Things that are easily houseruled in to give a 4E feel:
- Death and Dying concept.
- Action Points.
- Point Build for characters.
- Bloodied and Healing Surges.
SNIP
I actually do a fair amount of this with Action Points. Basically, I give the PCs 1-2 APs at the start of the session, and will reward up to 1 more for good roleplaying during the session (like, REALLY good roleplaying). They have 3 possible uses (you pick one at the time you spend the AP):
1. Immediate action; heal you for 1/2 of your normal max hp
2. Free action; give you an extra action that round (swift, move, standard)
3. Auto-succeed one roll you make (attack, crit confirmation roll, save, skill check, ability check)
I also use 4e design principles for crafting my monsters. Short-version is that I give them a little more HP on average (unless I"m using minions, then they act like 4e minions), and trim down their abilities and attacks into a suite of 2-6 "powers". An example of a goblin cleric of Orcus taken from the Necromancer Games module, Lost City of Barakus (plz don't sue me; this is a modified character!):
Grezzo, male goblin cleric of Orcus 4; CR 4
Small humanoid (goblinoid)
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Aura: Protection from Good; hedges out good outsider or summoned creatures
HP 56
AC 21, Touch 15, Flat-footed 18; +2 vs. good attacks
Fort +8, Ref +3, Will +7
Speed 5
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Smiting Mace (standard, at-will)
+9 melee; d6+5 damage

Light Crossbow (standard, at-will)
+6 ranged, d8/19-20 damage

Inflict Moderate Wounds (standard, at-will)
+6 melee touch; 2d8+4 damage; Fort DC 14 for half

Hold Person (standard; recharge 5, 6)
Range 12; target makes Will DC 16 or becomes paralyzed (save ends)

Cause Fear (standard, encounter)
Close blast 3 x 3 squares; All targets are shaken for 1 round; targets must make a Will DC 17 or become frightened for 1d4 rounds.
Squirrely Lil' Bastard (immediate; when missed by a melee attack)
The goblin can take a free 5-foot step.
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AL LE, Languages: Common, Goblin
Skills: Concentration +8
Str 14, Dex 14, Con 16, INt 10, Wis 15, Cha 10
Equip: +1 light mace, light crossbow w/ 15 bolts, breastplate, light wooden shield, potion of cure mod wounds (2d8+4)
For full-round attacks, I just do something like this:

Dogslicer (standard; at will)
+9 melee; 1d6+1/19-20 damage.

Shield Wall (full-round; at will)
Makes 1 Dogslicer attack, followed by a Shield Bash.
· Secondary Attack: Shield Bash +4 melee; d6+2 and target is pushed 1 square and must make a Reflex save (DC 14) or be knocked prone.
I use 4E language whenever possible because it's easier for me to read and comprehend quickly in the midst of combat. But I still use mostly 3E ranges, DCs, durations, areas of effect, etc.
You'll notice I took out all feats (any that affected his game stats were simply factored in), tossed out his spell list (any spells I assumed would be pre-cast were factored in), and I cranked up his attack bonuses just a tad (usually I add +1/2 level or so).
My enemies definitely are tougher than the CR would suggest, but I sorta eyeball it so that it's more like a CR appropriate creature is a tough encounter, rather than a standard encounter. And I don't send them up against anything more than 2 or 3 CRs above their level, tops (kinda like 4e, oddly enough). To counter this, I often reduce enemies' versatility, slow them down a little (for creatures that have like a 150' fly speed, like dragons), or make some of their nastier attacks dependent on their last nasty ones hitting...i.e., make it a secondary attack that requires a weaker "basic" attack (like a simple claw or bite attack) to hit before it can even be attempted.
Not sure if that makes a lot of sense, but I've been running like this for about 3 or 4 sessions and it's been awesome. Combats have been more dynamic, more edge-of-your-seat, and easier for me to keep track of. It takes me like 15 minutes to build a badguy or two, as opposed to 30+ min before.
All of this could completely break at high levels though, so know that I've only been doing this during levels 3 and 4 so far.