At first, forgetting we should look for mate in 4, this was my idea:
I haven't considered 1... Rc6 probably because I thought it was inferior. But I didn't even see the possibility for that mate, which is brilliant.
Like you say however, 1... Rc6's weakness is that it does not cover white's escape route on the d file.
From the beginning you can clearly see that black has a deadly hold with that knight, blocked pawns, and the help of the rook. Immediately patterns of many knight and rook checkmates come to mind - of course, black being a knight and a rook up doesn't leave white with much counterplay.
When I first arrived at this position, I thought the mate was forced. Then I noticed that there is a way out (and only one) for White. I'm still posting it though, because it's unusual to find a (nearly) unique solution in an actual game (and also because the final mate is rather nice).