I don't think that if you work this into a 3.xE game that you're just playing 4E. 3E is a lot more rules heavy (that's what I like about it) so simply adding in powers could never make it 4E. This is how I'm envisioning it for my game, it's still a work in progress, but these are the concepts I'm using:
1. I think you have to ditch the 3E model of multiclassing and use the 4E concept (use Feats to learn abilities of other classes, such as ability to cast magic and then a feat to learn a spell). Adding in powers, and then allowing people to take another class for 1 or 2 level, just to get powers, would seriously unbalance the game.
2. I'd only use powers for non-spellcasting classes (and possibly hybrids like Bards). I'd keep using the 3E spells for spellcasting classes, but change spell descriptions (such as eliminating save-or-die effects, make damage more level based, etc.), but make them at will (eliminating Vancian magic). Also break up spells into normal at-will spells and rituals (basically anything that takes more than one round to cast would be a ritual), and give spellcasters a different BAB progression for casting spells (different than their standard BAB for weapon attacks and more equivalent to the fighter BAB, to keep spellcaster effective at all levels). You don't have to change the spell levels, just change the spell charts for classes to list how many spells and rituals they gain per level, and what level spells/rituals they have access to at specific class levels. I'd also give them access to all 0 level spells at 1st level. You can also add Feats that allow classes to learn extra spells, rituals, or powers.
3. Eliminate multiple attacks (basically only one attack per round - however a power could still be a full round action), then figure out a progression for classes to gain powers (how many per level and what level powers). I'm specifically looking only for powers that I can explain or simulate a real effect. If the power is too abstract, I don't use it. To keep it simple, I plan on having all powers be encounter only (no daily or at-will). In my thinking it takes some work to set up a power (although I call them maneuvers), and once it's been used, the bad guys will be ready for it a second time (at least for the rest of the encounter - although I wouldn't be above allowing a player to use an action point* to reuse a power, but no more than one reuse per encounter). I'd give them a bonus to hit over and above their normal attack bonus, these should be special attacks that when used, usually hit and do substantial damage.
I like the idea of adding in powers for non-spellcasting classes because it will give them access to colorful, high-damage effects, just like spellcasters.
*I use action points also, but I'd been using these before 4E came out (I think I originally stole them from Star Wars).
(I'm still working on developing this for my game, but my project has been sidelined recently due to health issues.)