You just have to know what to look for. 4BT's are everywhere, many don't even know they have one.
IMO, a 4BT is a more useful engine than a 6BT. A 4BT will easily make more power than you can use in an 8000 pound pickup, even if you tow 10K up 6% grades regularly and it will always get atleast 20% better mileage (set up right more like 30%). Not to mention it weighs 2/3 as much, lasts just as long and fits in a smaller area. In that respect a 4BT deserves the price premium it gets.
Just make sure you're getting what you pay for. I've bought a lot of junker 4BT's for junker price, some junk, some fixers. I feel bad for anyone who buys crap off ebay for new price. A few years back I got screwed by a NY ebay seller paying good money for a "low mile" 4BT mated to a 4L80E that came with a controller and harness that was supposed to make it all function.
What I got was a likely 300K++ hard mile abused 4BT with a bad headgasket, bad head and junk injection pump rusted to oblivian with some fresh paint, a 4L80E with bad planetaries and a 350 gas 4L80E breadtruck computer and pieces of the 4L80E harness just tied in a knot and wrapped in electrical tape. Of coarse this didn't become aparrent until a couple months past the auction end when I had it ready to start.