I just realized that another reason I like 4TRK so much is that its workflow feels very similar to the Yamaha Seqtrak, which I own and absolutely love.
It has several drum and percussion tracks, three synth tracks, and one sample track.
What I don't like about the Seqtrak is its pattern system, the lack of a global audio channel, and the fact that it has no screen at all =(
Sadly, I'm selling it to fund a more versatile iPad.
But 4TRK looks like a great replacement: a similar workflow, but with a more user-friendly interface and a lot more flexibility.