Chess960: Opening Puzzle No. 4

Ok this one is pretty tricky! Let's say the difficulty level is high:

SP228 (NBBRQKNR): White to move: Find the only move with advantage

The answer is somewhere on this post. Now if you got the answer wrong, join the club! I simply did not see it because optically it looks wrong.

Enjoy 960!

1. Ke1! (only move) ... f5   (only move)

2. f3   (only move) ... Rd3! (only move)

3. fxe4 (only move) ... Rfd8 (only move)

4. Nc4!!(only move) ... RxR  (only move)

5. Kf2  (only move) ... Qxe4 (only move)

6. Rxd1 (only move) +/= 
(Feel free to post a comment if you do not understand)