I'd recommend more of a Two-handed weapon Fighter or Barbarian in lieu of using Two-Weapon Fighting.
There are multiple reasons to do so, such as the Accuracy penalties you get from Two-weapon fighting lowering your overall damage, being unable to utilize Power attack fully because of said accuracy penalty, and the fact that your gold is being spent on two weapons instead of just using all of your gold on a single weapon.
Also, Barbarian gets a D12 hit die against a Fighters D10, gets a full Base attack bonus progression, and their rage ability gives them bonuses to blanket will saves instead of just will saves against fear like the fighter, Along with a Substantial bonus to Strength and constitution when raging. ( Not to mention you probably aren't going to use all your rages in a day later in your game )
As far as your stats are concerned, for your TWF Fighter, You have the right of it. Strength, Dex, and Con are your most important stats, wisdom being secondary for will saves. Strength will contribute to your damage, Dex will boost your armor class, Con will increase your HP quite a bit.
You'll obviously want the Two-Weapon Fighting feat, So that's one of your feats. For your other two You're level four, so you could take Improved Initiative to improve your chances of getting the first action with your dexterity, and Jaguar Pounce so that if you do get the first action you have a pretty good chance of getting the jump on someone and using the high crit ranges on your light weapons to get a crit off when the fight starts.
I believe using a non-light weapon in your main hand, and a light weapon like a dagger or shortsword in your off-hand is still considered standard with TWF, But since you'd be using two weapons you'd want your weapons to threaten more, so a mainhand Scimitar to vary up the types of damage you deal ( Slashing v. Piercing ) while giving you a larger crit range on your main hand would be the best.
So I've got based on the stats specified: Str: 18, Dex: 16, Con:21, Wiz: 12, Int:10, Cha:10
Feats: Two-Weapon Fighting, Double Slice, Two-Weapon defense, Weapon Focus: Scimitar, Weapon Specialization: Scimitar, Jaguar Pounce, Improved Initiative
That puts you with 4d10+20+4HP, With a 1d6+6 Main hand. If your opponent is flat-footed your crit range is 15-20/x2, When they aren't its 18-20/x2. Your offhand is 1d6+4.
This should deal enough damage in the first round to get an enemy to focus on you instead of on someone else in your party should you roll a crit. Later on when you hit level eight you can retrain your Jaugar Pounce Feat into Improved Critical.
Some other feats you may want to look into as you level are Furious Focus, Shield of Swings, and Pushing Assault, These work well with Critical builds as if you push a target more than 5ft away, they cant use a full-attack action during that round if they don't have pounce.
Firstly, it sounds as though your players are doing a lot of the heavy lifting for you in that they are being very easily manipulated without much effort on your part at all. But let's talk about manipulation.
Pa-pa-pa-poker face
Manipulation relies on getting people to do something without them knowing exactly why you want them to do it, or even that you want them to do it at all sometimes. Why? Because if they knew your reasons, they most likely wouldn't do it, and that doesn't help you. So lying. Your duke should agree to pay these adventurers anything they want if they do the tasks required of them. And he will lie. Oh will he lie. He will lie gleefully and to their faces, especially if they are silly enough to just blithely believe everything he says to them.
But what if they don't believe him...
Believable lying for beginners!
The best lies rely on two key concepts.
- A kernel of truth
- An appeal to emotion.
Let's go through them.
1) The more outlandish the lie, the less likely that someone will believe it. This seems like a common-sense thing, but people have a hard time with it. The best way to lie, then, is to distort or stretch the truth. Maybe the kobolds really do fear the dragon and want it dead because it could kill them all. It just also happens that a dead dragon means its horde is ripe for plundering, but the kobolds won't mention that they plan on taking all of that treasure for themselves. Why bring it up?
And an important thing to remember here: if you're playing someone who is actually manipulative, they will not seem stereotypically manipulative or as though they are lying. No long pauses or sideways glances, no hemming and hawing over details, no emphasizing exact wording ("I won't take your treasure." etc.) Lie. Lie right to their faces. Straight faced and unabashedly. It's amazing what people will buy if you just don't call attention to yourself and your weird behaviors.
2) Appeal to emotions. The players must care about some things. A manipulative person can tap into those desires. If your party is obviously in it for the wealth, maybe the duke talks about all the hidden treasure that would be "too much of a hassle" for the kobolds to dig out themselves, but the heroes are welcome to any of it they find along the way. If the heroes are sympathetic to the plight of the underprivileged, perhaps the thing the duke wants them to do is causing famine or disease, or is hurting the poorest of the community. "Really, it would be better for everyone if someone stopped it, but darn it I just don't have the resources to." Say anything to get them to agree. That's the point.
You lying bastard!
When the PCs do find out they've been had, there are one of two ways the duke can play it. He can try the "it was a misunderstanding" approach if he hopes to manipulate them again, but more than likely he will just be dismissive. He presumably has a contingency of kobold soldiers at his disposal, an army should be more than enough to take care of any PCs. Into the arena pits they go! (The duke may want to ask for more soldiers from his emperor while the PCs are away. He knows he lied to them and knows that they'll be pissed and knows they were capable of killing a dragon.) Don't make it a big show either. If the PCs call him out for his lying ways, he just accepts it. No reason to care what they think, they aren't kobolds and so they are inferior.
Best Answer
The best overall source for Pathfinder character guides is Zenith Games' Comprehensive Pathfinder Classes Guide, which links to multiple Fighter and Ranger CharOp (character optimization) guides with sample builds. For builds in general, Paizo's NPC Codex is one of the best books ever and has a bunch of interesting fighter and ranger builds of every level (and Hero Lab has it as a data pack, woot!). Those tend to be more modestly optimized, but probably do a better job of showing the kinds of PCs that Paizo expects in their adventures.
However, you should stop to consider before doing that. High levels of character optimization bring a lot of problems to groups and GMs. If the players are happy with their own builds, trying to get them to "up their numbers" isn't necessarily welcome. Let them play the way they want. As the GM you can control the power of encounters they hit, so if they can't handle CR=level+3 encounters, just don't do that. With published adventures the CRs are set but you can easily control the rate of advancement - go to fast advancement and then non-optimized characters can meet the printed challenges (there's obviously 10 other ways of finessing that but that's the most basic). Paizo modules tend to assume non-super-optimized characters, as their sample iconic characters should make clear.
If they are veteran players, they likely know they could optimize and are choosing not to. You should get on the same page as they are with game expectations and then run the game the group wants. My recommendation is to take a couple sample characters to them and say "hey, you know in Pathfinder/3.5 you can make uber builds with really high damage" - and then either they say "Really? Awesome I love me maximizing my DPS, let's do it," in which case yay, except that then you have to figure out how to run for a super-swingy glass cannon high CharOp game. Or they look at you like you're a deviant, in which case you should rethink your life choices and play a non-op game!