1st - Some advice
I assume you are playing online, seeing as you have 4 people. I found that my chances of survival are all due to chance. Me and my friends usually go up the stairs to the chain gun, then across the tube to the right. With any luck, the area is littered with Molotovs. We usually sit up there and toss them periodically, but you've got to respond to smoker grabs pretty quickly. When supplies run out, head across the left tube from the chain gun and use the supplies there. They seem to last the entire round. As for the tanks, well, just have one person as the designated Molotov-holder, and remember to use gas tanks.
EDIT: And, if it helps, have some people shimmy onto the ledge behind the building's outcrop when on the ledge to the right of the chain gun. This will prevent friendly-fire incidents and will get a few people out of harm's way. Use the beat-down methos to free constricted/pounced teammates. I recommend the auto-shotty at all times, but on expert, you really have to watch your shots. Let the zombies come up to your ledge, and as they climb, remove their heads.
2nd - A complaint
Can we stop re-posting Iwm's comment? Not only is it off-topic, but when TWENTY PEOPLE DECIDE TO RE-POST THE EXACT TEXT, the thread gets screwed. People, epecially me, don't enjoy scrolling through 2 or 3 pages of the same text over and over and over.