Ok my basement floor drian...again we had heavy snow melt and rain and its wants to back up...I have an anti back valve so it cant come on the floor...the drain is 4" cast and has a trap below it,,,right behind the drain is a cleanout that sticks up about 3" and is capped...I took the cap off and t burped and the water level dropped.....

Is that normal ? Should I leave that cap off ??

i would think under ordinary circumstances the floor drain will flow just fine. it is 4" pipe and with water running across the floor and down the drain, the 4" pipe will never fill up so there will be plenty of air for a free flow.
BUT, you have a pipe full of water because of the melting snow and rain so when you opened the cleanout you "burped" enough air in to let it drain.

any downside to leaving the cap open ?

If the water exceeds the ID of the 4 inch line, it will vent thru the standpipe without its cover.

Best Wishes

so thats a good thing correct ?? Can water come up thru the vent if I leave the cap off ?

yes, water can back out of the open pipe. you can always extend that pipe during the times of year that are a problem.

well the cleanout sticks up now about 5"