I was thinking about this too, so here's my build (added an additional Feat from your houserules):

Lvl 4 Tiefling, Archfey Tome Warlock

This build points to forgive the melee and AC options for a more focused caster. I liked your idea of an hybrid, but seems to me that it really needs to be an hexblade, going in the direction of the other classical builds around. Tiefling adds some interesting features, while allowing to reach 20 CHA on lvl 4, increasing the overall effectiveness and chance to hit of the few spells you can cast.

By Lvl 4 you have:

• Cantrips: Eldritch Blast, Minor Illusion, Vicious Mockery, Thorn Whip, Shocking Grasp, Thaumaturgy, Friends
• Lvl 1 Spells: Hex, Sleep, Faerie Fire
• Lvl 2 Spells: Hold Person, Phantasmal Force
• Hellish Rebuke 1\day (2d10)
• Fire Resistance
• Agonizing and Repelling Blast

I chose Archfey over Old One for several reasons:

• Spells: Dissonant Whispers and Tasha's seems kinda redundant to me, especially when you can get Hold Person at lvl 4. Sleep it's an excellent spell vs. low level mass enemies, it has no concentration and grows with spell slots. Faerie Fire can grant advantage to the entire party vs one or more hard targets. Both pacts have Phantasmal Force which is a great disable for low int enemies and fun to use too.
• Pact Powers: Fey Presence is an area disable which fits more the theme than Telepathy granted by Old One. At lvl6 Entropic ward is fine, but Misty Escape is simply awesome.

Invocations: I'd go to PimpMyBlast just because this is your only reliable source of damage in a lot of situations where CC or forced movement are not options - If all CCs fails, if you want to blast that minion in the backrows not worthy a spell, in long fights, ecc.. But if you like, you can forgo one of the blast invocations to get rituals, which can give you a bit more flexibility and a Familiar.

Feats: i'd really like going first at all costs with this build, so i'll take Alert. A well placed Sleep or Hold person the first round is no joke. Otherwise Moderately Armored can give you better ac with a shield, but going melee will require War Caster too to be a viable option.

Ability scores in order of importance: Max Cha, High Dex, High Con, Mid Wis, Low Int, Low Str

Role: cc\secondary caster

Playstyle: Disable high treat enemies as soon as you can. Use forced movement (Blast, Thorn Whip) to generate AO from allies and reposition enemies at your vantage. Minor Illusion to modify battlefield (covers, false gaps, ecc), Vicious Mockery to help ally, Shocking Grasp to disengage, Thaumaturgy\Friends as social tool.

Progression: At lvl5 you can get the infamous Devil's Sight + Darkness combo which can transform you into a walking nightmare of disabling madness, very hard to target. Lots of tactics here, disabling sight with plain movement is a powerful option, plus you attack with advantage; just take care of LOS of your allies. If you want more utility you can always get Rituals. Darkness is 1/day from Tiefling so you can also get a lvl 3 spell. While patron's spells are not that great, at lvl 3 you have 2 strong area disables (Fear and Hypnotic Pattern) and Counterspell, which is really useful if you are the only caster in the party.

Here is Lvl detailed progression:

lvl 1
Tiefling: +2 Cha, +1 Int, Thaumaturgy
Patron Fey: 1-turn Area disable 
Spells: 0 - Eldritch Blast, Minor Illusion; 1 - Hex, Sleep

lvl 2
Invocations: Agonizing + Repelling
Spells: 1 - Faerie Fire

lvl 3
Pact of the Tome: Vicious Mockery, Thorn Whip, Shocking Grasp
Spells: 2 - Phantasmal Force
Tiefling: Hellish Rebuke 1/day

lvl 4
Spells: 0 - Friends; 2 - Hold Person
Feats: Alert (from houserule 4th lvl)

A single-level dip in cleric can get you three things: the Inquisition domain with its +4 to dispel checks, Turn Undead that can power Divine Defiance, and the ability to activate a prayer bead of karma for a straight +4 bonus to CL.

Ur-priest does get you the last two things, but it’s difficult to enter and you need two levels instead of just one to get Rebuke Undead from it.

Anyway, Divine Defiance is a feat in Fiendish Codex II: Tyrants of the Nine Hells, and it lets you burn a Turn Undead usage to attempt a counterspell as an immediate action. Obviously valuable.

In general, the cloistered cleric variant is a great 1st level to take for a lot of people: you get 4×(6+Int) skills, much better than you would as a warlock, you get all of the above, another domain, Knowledge Devotion from Complete Champion, and you get identify as a 1st-level divine spell, i.e. you don’t need the arcane material component for it. The Undeath domain can get you Extra Turning if you want it, or you can use the other domain to get all manner of excellent things. Check out the Dipping Cleric Handbook for more ideas.

Divine Defiance requires “divine caster level 3rd,” which warlock doesn’t qualify for. The solution to that is Practiced Spellcaster (cleric) from Complete Mage; that will get your cleric caster level (but not any other aspect of cleric spellcasting) up to 5th to meet the requirement. You will also most likely want Practiced Spellcaster (warlock) to get back the CL lost when dipping cleric, as that would otherwise hurt your dispel checks.

The eldritch disciple prestige class from Complete Mage could progress your cleric spellcasting along with your warlock invocations, but it’s not that great and it would require you to take another two levels of cleric to get the 2nd-level divine spells it requires. Not worth it. Shenanigans can get you in without those levels of cleric, and that might be worth it, but if you want to go that route I recommend just asking your DM to adjust the class’s requirements to begin with, rather than get into shenanigans. For example, eldritch disciple expects a Cleric 3/Warlock 1 entry, and then at 1st level gives you invocations only (so you are effectively a Cleric 3/Warlock 2 at 5th level) and from then on progresses both. Maybe your DM would be willing to flip it, allowing you to enter as a Cleric 1/Warlock 3 and then progressing spells only at 1st (so you are effectively Cleric 2/Warlock 3).

The other prestige class worth mentioning for any warlock is chameleon from Races of Destiny. The second-level bonus feat can be changed every day, which means it can be any invocation you need that day (by making it Extra Invocation), or if you get far enough, an Item Creation feat to go with your Imbue Item class feature. This makes Cloistered Cleric 1/Warlock 9/Chameleon 2 a pretty strong build for you, and when you hit Cleric 1/Warlock 12/Chameleon 2 at 15th, you’ll be exceptionally versatile.

I will mention, however, that I have done a counterspelling warlock quite similar to the one I describe above, and while it works more or less, it’s pretty boring. If you are really interested in a high-power spin on the idea of ruining other casters’ day, what you really want is a Black Tactica build: use the war weaver prestige class to buff all of your allies as a move action at the start of combat, and then spend your actions in combat readying to target anyone who starts casting a spell. You don’t ready a counterspell, however: you ready the most reliable single-target nuke you have. In addition to hitting the spelcaster hard, which is painful, this also forces a Concentration check with DC 10 + the damage dealt. It’s trivial to pump the damage well beyond what anyone can hope to hit with the skill check, which means they will lose the spell.

This is a very high-power build that may actually make your game less fun, as your enemies start using similar tactics and combat becomes something of a deadlock, everyone readying nukes against the others, and whoever dares to move first still not getting anything done.