Way to distinguish OLL3 from OLL4

3 side faces have U-face-color block at the right side, and 1 side face has that colored edge -> OLL3

3 side faces have U-face-color block at the left side, and 1 side face has that colored edge -> OLL4

So, after F2L finished, in the 75% of the case, you see U-face-color block on the F-face and recognize which OLL is.

If you happen to encounter one colored edge on the F-face (25% chance), you can recognize them as follow.

A corner dot on the U-face is at the down right corner -> OLL3, or down left corner -> OLL4.