The joy and limitation of 30 Rock is that it knows no business like show business.

Returning Thursday night for its fourth season, this three-time Emmy winner may be the sharpest, most cutting spoof of network TV that TV has ever produced — which is why it's more popular with people who work in TV than with the viewing public as a whole. The show even uses that disconnect for one of its first and best jokes, as Jack (the invaluable Alec Baldwin) gives the folks at the fictional TGS skit show a pox-on-both-houses pep talk about broadening their show's appeal.

"We'll trick those race-car-loving wide-loads into watching your lefty, homoerotic propaganda hour yet."

Not yet, Jack, at least not to judge from the first two episodes. As funny as this scattershot sitcom can be, the problem remains that show business (and in particular, GE's business) is a topic of limited long-haul mass appeal. It can lead to some great, if random jokes. But with rare exceptions, from last season's trip to Liz's high school reunion to any of Jack's encounters with his mother, 30 Rock falters when it leaves the business behind.

If it has trouble finding enough for Jack and Liz (Tina Fey) to do, it collapses when it goes beyond them. Jack McBrayer is a joy as Kenneth, but a little of that joy goes a long way. Jane Krakowski's Jenna is a joke in search of a character, and Tracy Morgan's Tracy is, well, a matter of personal preference. You either find his wandered-in-from-outer-space act entertaining, or you don't.

As for the rest of the cast, if they all traded roles next week, would you care?

Think of how precise the characters were on the last sitcom with an Emmy-winning streak, Frasier, which won five, and compare them to the cardboard cutouts of 30 Rock. It's a good series, but you can't help thinking that its Emmy three-peat says more about the academy, and the state of the sitcom, than it does about 30 Rock.

Still, that state is changing. Curb Your Enthusiasm is doing showbiz humor just as well, and possibly better, with its Seinfeld reunion. And other shows have moved past the binds of absurdity to deal with real people and real problems again, from the relationship complications of Cougar Town and The Big Bang Theory, to the family and economic woes of Modern Family and The Middle.

If 30 Rock wants to defend the Emmy, it had better up its game. And expand it.