Try a Dash-8 Q400!
That would be great if you want to experience a gear collapse up close and personal
Besides, the Qrash-400 doesn't come cheap.
Saab 340B has a maximum payload of around 3.9 tons, might be worth having a look at, though I appreciate it's 1 ton short of your target. But it's a very rugged machine, and since Mesaba have anncounced they're dumping all theirs, they might be available at favourable rates. Try contacting RAF Avia and see what they can offer.
Another option could be the Fokker-50, if you can find one. I believe Farnair operates a few, you could try them. Don't know if a Fokker-27 could do the job, but try MiniLiner and see what they have to say about it.