Batter hits a deep fly ball at the right field fence.
the right fielder leaps up and catches it, his momentum takes him over the 300 fence he hits the ground as he brings his glove up to show the field umpire the ball falls out of his glove umpire calls 4 base error.
correct call?

yes.. read the note part at bottom of CATCH definition

1.10 • CATCH
A catch is a legally caught ball that occurs when the fielder catches a batted or
thrown ball with his hand(s) or glove and the resulting release of the ball must
be voluntary. NOTE: It is not a catch if a fielder, after he contacts the ball,
collides with another player, umpire or a fence, or falls to the ground and loses
possession of the ball as a result of the collision or falling to the ground.

Correct call ... The umpire's call appears to hinge on his judgment (key word) that the fielder did not complete the catch ... That's a judgment (there's that word again) call not subject to appeal ... I've got a four-base award ...

I agree with DD on it being a judgment call as is every call.. what I would be focused on is whether the release of the ball from the glove was “voluntary” .. unlike football the ground can cause a fumble in softball