I've tried several times, but failed, to uncover any cards that reveal a black four.

Hi Harleyman -- I'm no expert but, for what it's worth, maybe this helps. One black 4 is face up on a pile. The other three fours (one black, two red) are all at the bottoms of the piles on the far right and the second from right. So the way I could finish was to play so that I could get to the bottom of those piles. That's done by following the clues for the game. For me, I had to wait for the 7D, 5S etc. Hope this helps.