The intersection of the 3 on the right and the diagonal 6 on the left is important.

引用自 JorgeJunior：

The intersection of the 3 on the right and the diagonal 6 on the left is important.

Care to elaborate, I tested both a blue or blank there and both seem to work, it doesn't seem to force any of the other {4} squares either

It's black.
If it's blue, you won't be able to solve the {4}.

I'm interested in how you figured the two by the {4} prior to posting here? I'm trying the puzzle and don't see what leads to that conclusion.

There are four known blues on the 6 crosser, with an implied 5th at the remaining two unknowns around the black four on the crosser above. So if the 3/6 intersection is blue, then both the 3 and 6 crossers are satisfied, but that means there would be three black cells around the {4}.