demon's souls is a must

for a 4th game i'd go with either killzone 2 or mgs4

mgs4 if you want more single player because you can have fun beating it 3 times at least, even tho i put 100s of hours into online
killzone2 if you want more multiplayer even tho single player is a decent length.

Demons Souls!!!

little big planet i have given that game atleast 120 hours it has immense replay value
U should also check out infamous its good and has good replay aswell

I think inFAMOUS is an awesome game. It's my personal third favorite game of all time, only below Metal Gear Solid 4 and Uncharted 2. inFAMOUS I think took me about 20 hrs to beat bc i did all the side missions. U can choose to be good OR evil; and the story AND your powers change depending on which u choose to be. I think it's def worth it to play through the game once as good and another time as evil, so this puts ur playtime with the game about 30-40 hrs I'd say.
Moving on, speaking of MGS4, you GOTTA get this game imo. I think the campaign is like 20-30 hrs long for one play through. This game is epic in every way in my and many other's opinions. A lot of ppl, myself included, consider it their favorite game of all time.
Another game I think u have to get is LittleBigPlanet. This game is SO fun! It may come off as a kid's game at first, but the amazing thing is that it is so fun and ingenious that all age groups can enjoy it. Over this past summer, my friends and I got together on a regular basis JUST to hang out and play this game (we're all 22 & 23yrs old btw...). My one friend had a 360 and still all he wanted to play when we hung out was LittleBigPlanet. Now he just asked for a ps3 almost entirely bc of LittleBigPlanet. If u don't know much about this game, look it up. I think u can get it now for like $20-30.
Anyways, there's a crap-ton of great PS3 exclusives, and this is why i love the system so much. Other great exclusives are: Killzone 2, Resistance 1 (awesome campaign plus good multiplayer) & Resistance 2 (good campaign plus AWESOME online 8-player coop, plus up to 60-player multiplayer matches), Ratchet & Clank 1 & 2, Uncharted 1, Motorstorm 1 (Motorstorm 2 has 4-player split screen, but just wasn't as fun to me as Motorstorm 1).
There's also a bunch of upcoming exclusives: God of War III, Heavy Rain (looks pretty freagin sweet), MAG (256-player online matches), The Last Guardian, Agent (exclusive being made by makers of Grand Theft Auto series), Twisted Metal PS3, White Knight Chronicles (if u like rpg's).
This is probably a lot more than u were looking for, but I figured better to give u too much than not enough.

Here's a thread that might help you to find a long game:

http://vgchartz.com/forum/thread.php?id=80065&page=1

Depends on what you like...

Demon's Souls
Valkyria Chronicles
Folklore
MGS4
PixelJunk Monsters!!!! =D =D Yeah, really, this game IS time consuming!
Yakuza 3 will arrive soon. I spent 70 hours on Yakuza 2 wandering around doing all side-missions, etc...

There are more, but I have to bounce!

Demon's Souls, InFamous, KZ2. If you want to wait, Ar Tonelico 3, WKC, MAG, and maybe GOW3 (if it does at least an 8 hour campaign on normal and then you have to go through god and titan modes for trophies, costumes, possibly urns, etc).

valkyria chronicles- amazing story and good gameplay, also nice original visuals a must IMO

Oblivion- NOT an exclusive but is a long game

demons souls- really hard and will take some time getting used to it if your not ready for the challenge

MGS4- will take some time

disgaea3- not the best visuals just the traditional disgaea visuals but full of things to do.

killzone 2- great FPS with really good graphics if you include the multiplayer then i guess youll be on it some time

LittleBigPlanet- if you actually get into this you'll spend some hours getting creative with it

plus you got a load of rpgs coming out this year like;
final fantasy 13
final fantasy versus 13
last rebellion
Tales of Vesperia (hopefully will get wetern release this year)
White knight chronicles
yakuza 3

Hmmmm long SP?

Valkyria
Infamous
Demon Souls
Resistance FOM

personal recomendations for you

Ratchet and clank TOD and AciT
LBP
Heavenly sword
MGS4
Motorstorm PR
KZ2