Given the following assumptions:
- You have engaged in RPing before
- You want to learn combat
- Your friends have also RPed
- You own a copy of the DMG
My recommendation is to not actually start out with an adventure. It sounds like your primary intent here is to learn the combat system. In the back of the DMG, there are descriptions of how to build a random dungeon and how to build random encounters. If you and four of your friends can create an adventuring party, you may learn the combat system (in quite an entertaining way, I must say, having done so myself) through a random dungeon and random encounters in said dungeon. Someone may choose to control the monsters every round, and it's worthwhile to have the duty of controlling them rotate around the people who are willing: that way everyone can start modelling in their head how monsters think and operate.
Once you've gotten a handle on the monsters and combat, then is the time to run adventures. My recommendation would be to use the chaos scar adventures and encounters from DDI, as they represent much later adventure design and this design will be similiar to that run in D&D Encounters at a game shop. Once you've run through those, test your skills with a fourthcore run of the "FourthCore Weekly Grind" which can teach you really important lessons about manipulating and using your environment that can be quite applicable in an Encounters game.
Choose 1 of the following: Individualized Plot Decisions, 4th Ed Party Cohesion, Respect Characters Ideals1. At the end of the day, your system and your goals don't match up well.
You can:
- Present challenging decisions
- Prevent PC conflict
- Respect different ideas and ideals.
- Run a game designed for murderhobos to have colourable excuses to kill everything.
At the end of the day, your choices oppose each other and are not particularly compatible with the game system you've chosen. If you want to present choices that matter, they have to matter: PC conflict must be allowed as the losing side must be allowed to escalate (see Dogs in the Vineyard).
While the same page tool is useful here, it's important to begin by articulating your priority of desires. If you want to require group cohesion, set up narrative (and if you can) mechanical incentives for consensus. If you want to present challenging decisions, figure out your characters' motivations and pry at them. You may find that your game of D&D doesn't fulfill your requirements well. This is fine. There are hundreds of game systems out there, and one of them will fit better.
Fourth edition can certainly present morally complicated decisions, but doesn't fufill your requirements as the decisions exist purely in the narrative area of the game. Therefore, they're an excuse to frame different set-piece battles, not have the possibility for party-conflict.
The way to think about it is that 4e is like a big summer blockbuster. There's plot, and there are action sequences. The plot does an absolutely necessary job of tying the action sequences together and making us care about them. In "plot" time, there are very little rules and the protagonists can shout at each other as much as they want... so long as the issues are resolved (to a point where they can banter with each other about them during the fighting at least) by the next "set piece battle."
4e's rules focus on set piece battles between the party and monsters. This conflict is built deeply into the game (mainly due to different rules for monsters and PCs). This means that PCs cannot engage in PvP: there are no rules for it. While you can fake it, fparty-conflict has never worked when I've tried it, save when one person betrays the party, discards their character sheet, and that character becomes a boss-monster for the party to kill.
Note the assumptions there. Things that interact with the party, by definition, die. Good encounter design in 4e is to threaten the party with death without actually killing the party. Note also the protagonist is the party not the individual characters within the party. More so than in all the other D&D games, parties are cohesive groups by the mechanics, and not simply a bunch of fellow travellers.
If you don't want a game full of tactically interesting set piece battles connected by however much plot you're willing to provide and your group is willing to engage in, 4e is not for you. The party, as protagonist, can certainly make these ethically challenging decisions: they inform which battles are to come and what framing goes on in those battles. The PCs can influence the party's decision making process, but cannot defect from it save by defecting from the game.
1On a 10+ choose 2, On a miss, the MC gets to completely derail your campaign.
Best Answer
One big issue with not having a full deck to play with is party/class/roll synergy. It's going to be infinitely more difficult for your players if, for example, they all roll Controllers, so they should think about what rolls should be covered and what you could do without.
DMG, page 31 has a nice little excerpt about that;
That isn't to say you have to have a Defender and Leader, although it would be a good idea, but they need to consider what they are giving up for each roll that they don't cover.
Character's Skills might also be worth mentioning here as well. If they all, more or less, choose the same skill set they're going to have a tough time with skill challenges.
So that's something for the players to think about.
Now for what you can do as the DM. You have a couple of options here.
One thing you can do is use the XP budget table on page 57 of the DMG to adjust each encounter to compensate for the lack of PCs. Instead of adding monsters to fill your XP budget, you remove some. You don't have to hit the exact value given, just as long as you're in the ball park you should be good.
For instance, your XP budget for 3 level 1 PCs would be 300, so for any level 1 encounters in your adventure you would need to remove monsters until the total XP value of the remaining ones is roughly 300.
Another thing would be leave the encounters as they are but stagger the amount of monsters on the battlefield, have a some tied up with some other task and join in the next round or two. If you have a large battlefield you can simply put some on the very edges so it takes them a round or two to even reach the PCs.
In the case of skill challenges I'm not too sure. You could maybe try reducing the complexity level by 1. It depends on the diversity of your player's chosen skills as to whether or not you think they should have a chance of success if you leave it or change it.