Hunting regs, page 33, Identifying a legal moose antler:
A point or tine is an antler projection at least one inch long, and longer than it is wide, with the width measured one inch or more from the tip.
Without a tape my first guess is that is not a 4 brow tine bull. I think that is my second guess too.
I would definitely walk away from it in a 4 brow tine area. My thought is, if you have doubt than don't pull the trigger. I don't like that feeling and it's not worth the mistake of shooting a sub legal animal.
I don't think it legally has four brow tines and I don't think that it is a 50" rack. I would not shoot it. Last year we took a bull with 3 brow tines and a bull with 4 brow tines in a 3 brow tine 50" area and both were 48". Both looked like 50-55" bulls but we would not have taken them without the brow tines. We passed two more bulls with two brow tines that were 46-55" bulls and ended up taking a 68" 229 B&C bull. Sometimes it pays off to wait for a better one.