Beautiful mate in 4

A friend of mine showed me this gem on Friday. He said that it took GM Ben Finegold one hour to solve, must have been mentioned in a video. I found the solution within say 7 minutes without moving the pieces.

It is so much more fun finding it yourself, so just give it a whirl! #4

Nice one, I hadn't seen this one before.

thought i have it in three seconds then i realized other promotion possibilities lol

I timed myself and got 7 minutes too. :P Coincidence? It's a nice idea.

Also just an irrelevant remark, the problem would be badly cooked if it were mirrored left-to-right.

Very nice. I did not find the solution.

@Illion, indeed, a mate in 3 with multiple keys... :-/

I got it in about a minute, but it's a #5. Is this correct?

1. Ne4 c3 2. Nd6 c2+ 3. Ke2 c1=N+ 4. Kf3 Nd3 5. Nf7#

Alternatively ...c1=Q 4. Nf7# which might have been the #4 referred to.

People, when they say mate in n moves and you emerge with a mate in n+1: si tacuisses. ;) There are numerous mates in 5.

And there's always ONE solution line, double keys are a serious flaw.

Nice one.

Here's the video:

youtu.be/qlX_806ZwOk?t=48m8s