A good question, and unfortunately one I'm getting more and more dubious about. With Telltale's current workload, we're definitely not going to see a Season 4 anytime soon. Very late 2012-mid 2013 at the earliest. Unfortunately, Season 4 is beginning to seem more like a matter of "if" than a matter of "when."
If I was asked this immediately after The Devil's Playhouse ended, I'd say "Of course they're going to make a Season 4! Sam and Max is Telltale's biggest franchise!" But that's no longer the case. Maybe I'm reading this article wrong, but
Back to the Future is now Telltale's most successful franchise. And if "most successful" means "most profitable," then Back to the Future has outsold all three Sam and Max seasons. Combined. Before BttF had even finished.
Telltale could make a Sam and Max Season 4, but they could also make BttF Season 2 (or another more mainstream project) and make far more money from it. Telltale hasn't given up on doing nichier titles yet (Fables and King's Quest are far from mainstream), but I'd expect them to start moving further and further away from that direction.
Money isn't just the only issue. Now, just to be clear, I don't work for Telltale and I don't know what goes on behind the scenes. Most of what I say is just guessing. I could be wrong, and if I am, I'm sorry. But I sort of get the feeling that Telltale wants to move away from sillier projects like Sam and Max. You'll notice they've been steadily inching towards more serious storytelling. Tales of Monkey Island, The Devil's Playhouse, Puzzle Agent, Back to the Future...you should be able to see the pattern. And Jurassic Park, The Walking Dead, and Law and Order all aren't exactly famous for comedy. If Telltale wants to be taken more seriously, that's understandable. Creative people (and game designers are creative people, unless they work for Activision

) shouldn't be forced to do the same thing again and again just because it's what people want. But that does hurt the chances of a Season 4.
There's also the chance of Steve Purcell making a new comic, though. Sam and Max are Steve's babies, and he'd definitely never get sick of them. However, Pixar seems to be keeping him pretty busy. He's apparently trying to work on the long-unfinished comic where Max gets shot, but...well, when you have to choose between your job and a side project, you go with the one that pays the bills.
Anyway, best thing to do is cross your fingers and hope the Freelance Police return soon.