I'd have to recommend OSRIC. Like somebody else noted, the reliance of 4e's "multiple balanced encounters per day" and the general slowing down of combat means that if you're going to let players get in problems that are way over their head, then you're left with three options, really.
1) They slowly and frustratingly get killed in a three hour combat
2) They fight one thing a day, then rest, leading to the "lovely" 15 minute adventuring day
3) Your games consist of endless easy combats
Really, I'd go for Labyrinth Lord / OSRIC / Swords and Wizardry. Leaning towards OSRIC, of course, if your players are used to 4th edition, as it can be hard to get used to -not- leaning on the rulebooks for everything. The advantage of the lighter frameworks, as you know, is that you can tinker much more and still have a game that works more or less like it should.
For what it's worth, I'm working on a *FREE* Labyrinth Lord player companion based on my own personal notes of what I'd like to play in a game, and there's no Vancian magic or anything else per-day. You could always take a look at that and steal from it, if you like. The example I'll point to is the
Sorcerer class I posted today. If that has the sort of feel you like, the player companion should be out soon.