You can check out http://www.arba.net
and look under breed photos to find pictures of all the different rabbit breeds (so maybe you can figure out what yours is).
It's good to here that he is eating well. Is he eating any hay, by the way? Try introducing some timothy hay or oat hay into his diet (these hays are high in fiber and act as a roughage and aid in the prevention and treatment of GI stasis, which is an intestinal blockage). Timothy is better, but you can use other grass hays if you do no have it available to you. If he seems like he's under 6-months-old (you can ask a vet to help you with determining his approximate age), then you can feed him a handful of afalfa hay everyday (high in calcium and protein and promotes healthy bone growth, however, for older rabbits over 6 months, it can cause bladder and kidney problems).
Just give him more time to adjust. He's only known you four days, so its okay if he's a little jumpy still. To help encourage bonding between you guys, try these things:
1. Sit in the middle of the floor with him running around and just sit very still and let him come up to you (this may take a bit). When he does, don't make any sudden moves, but lut him sniff you.
2. Hold him for at least 5-10 mintues straight everyday (depending on the amount of free time you have). During the holding time, you can talk softly to him, stroke his nose, gently rock him and even play some soft classical music in the background (animals like that kind of music).
3. Get a small piece of a really yummy, strong smelling, irresistable food (for most rabbits, bananas fit this category.... it drives them NUTS). Sit on the floor and hold it out (blowing on it in the rabbits direction may help him smell it better if he doesn't seem to notice it). Chances are, your rabbit will run up to you and go for the banana. When he does, hold on tight to the piece of banan... do not let him take the whole thing and run off under the bed. Instead, hold on to it (this 'forces' him to stay next to you and eat it). If he doesn't like the banana, you can try it with some other fruits and veggies (run whatever food you chose by the group first if you aren't sure if rabbits are allow to have them).
One last thing, do you know if he is neutered? If he is an older, unneutered male (over 4-6 months), then his testicles should be *very* apparent. You may want to check with his old owner to see if they had him neutered. Neutering can help sweeten up a rabbits attitude. However, I suggest trying to bond with him first and give him a couple more days to settle in.