Also it's not very hard to play expert by yourself because that means you get 3 aimbotting teammates that can shoot through each other and yourself with no worries of friendly fire. Expert with 4 humans (and everyone surviving to the end) is definitely more difficult.
I don't know who you're playing with, but having played Expert almost nightly for the past 3+ years with a large number of people, unless they are totally incompetent or TKers, the AI/bots are almost ALWAYS inferior.
-The AI cannot use grenades (mollys or pipes)
-Cannot use items (propane or gas tanks)
-Cannot form up coherently in choke points
-Cannot take cover in 'intelligent' areas during events
-Is utterly USELESS against Tanks (huge in Expert)
-Sucks at rescuing you from Smokers (often BACK UP towards you while shooting to melee you free, rather than shooting the smoker itself to free you)
-Often times runs through fire (incapping or killing itself in the process)
-COMPLETELY mismanages health packs and pills (often healing, and therefore wasting a kit if their health dips below 45, an ultimate expert No-no)
...and just generally fucking things up.
Yeah, they can't hurt you (with gunfire), or each other, and you can get through the first couple of areas mostly fine with them tagging along, and sacrificing them to move on. I know, I've done it. I'll play SP when no one is online. But beating the finale of something like Blood Harvest or No Mercy or Death Toll with only AI teammates on Expert, without using some kind of trick or glitch, is virtually impossible.