Good games under 4 hours long

Try Brothers: A tale of two sons

Hotline Miami (got a kickin' soundtrack too!) and FTL maybe?
I hear Nidhogg is only 30 minutes :P, looks like arse though...

The Stanley Parable is a really short, but great game! Give it a shot.

The Stanley Parable

Little Inferno

Fez

Thomas was alone

To the moon (Watched a playthrough)

VVVVV (Watched a playthrough)

Stick to the Man. I completed it in 5 hours but I've seen players complete it in 4. Even if it takes you 5 hours, it's cool and fun game to play

Finding Teddy
Brothers: A Tale Of Two Sons
Mark Of The Ninja (possibly)

You get an achivement if you beat Magicka in less than 4 hours.

Did not read all posts, but I'll recommend the BIT.TRIP games and Electronic Super Joy.

Shelter is a really intense expreience, about 1.5 hours.

wahhhh, who bumped this? Now my reply page is flooded :P

I'd say FTL, although I don't know about the "completing" part.

War In The Pacific Admiral Edition. Wait, no you said hours, not years.

braid and machinarium? they're around 5 hours i think

Brothers - A Tale of Two Sons
3 hours ish. But amazing story. nice graphics too.

Electronic Super Joy and Nihilumbra are two great platformers :)
They're really different, but they're both within your time limit (I think), and I enjoyed them both quite a bit.

Hotline Miami - 4H (except you're me, then you'll take much longer)
Cry of Fear (Free on Steam) - 6,5H/2,5H on second playthrough (i did so terrible, it also crashed sometimes and i had to replay some long parts, the average user will take roughly 3-4H to complete on the first playthrough.)
The Binding of Isaac - 0,5-1,5H (for the first of 8235731097531 playthroughs)

i tend to squeeze every second of gameplay out of games possible, that's why i've managed to pile up 40+ hours on Cry of Fear, for example.

Dysfunctional Systems: Learning to Manage Chaos (can 100% this game in less than 4 hours actually)
Long Live the Queen (each playthrough takes a few hours tops, does take multiple to complete)
Dracula: Love Kills (was about 3 hours for one playthrough, you'll need two to 100%)

And thanks to whoever necroed this one, was looking for some games that are short as well!

Mirror's Edge

I didn't read through all the comments but I've just finished "Brothers - A tale of two sons". Yes, just now in one session (~3h).
I know it's not everyone's taste, but I must say, I love the game. Although the story is a bit predictable, I especially love the artwork. Wonderful.

You guys know about HowLongtoBeat? Great site to get info about this kind of query...

(Edit: HLTB seems to be common knowledge in the thread. Mind me not.)

On a more personal note, here are some short ones I enjoyed (time; my personal grade out of 100):

1. Portal (4h; 99)
2. Tiny and Big (3h; 88)
3. Limbo (4h; 84 -- oh, you had that)
4. Jamestown (3h; 83)
5. VVVVVV (3h; 80 -- oh, that too :))
6. Offspring Fling (3h; 76)
7. Frog Fractions (1h; 72)
8. Samorost + Samorost 2 (1h30m; 70)

Thanks to this thread I completed Thomas Was Alone and got 100% on achievements.

Awesome.

Contrast (about 4 hours to complete all achievements)
I really love the atmosphere.

Gunman Clive - 1 hour (Duck Mode takes maybe an additional 1-2 hours).

Freedom Fall - 1.5-2 hours.

Tiny & Big
Portal

Just finished Papers, Please in a 2.5 - 3.5 hr session and loved it.