don't wonder off from the other 3 because you will just start another attack that otherwise wouldn't have happened. Take notice of other peoples health and heal them if you can, unless you need it yourself. At each safe room, try and get a health pack, use it, then pick up another if possible. Wait untill the others are done picking up what they need before you get the 2nd. Make use of the melee attack, it's very useful if you need to reload and are overcome with zombies.
One thing I've wondered myself is about flash-lights. No one seems to have theirs on so either it's bad to have it on OR you just don't see other peoples.