I have used braid before, 20 pd power pro, which I guess is the same as a 4pd diameter mono line. I have always used  about 30 yds of 4 pd mono backing on this with a uni knot. I have recently changed out to a 50 pd braid and seem to be getting  rats nests ( wind knots) when casting. Do I need a heavier mono backing, or is there a different knot I shout use?  Thanks

I think you need to adjust your reel. the 50# line is coming off the spool slower than the 20#. If the reel was set for the 20# and never adjusted for the 50# the spool is over spinning causing the backlash.

I use 50# spiderwire stealth with a 10# or 8# backing and have no problems

I don't think the choice of mono matters but I could be wrong.  I find it strange to in that going from 20 to 50# braid should have made wind knots and casting problems less of an issue. :-?

I would be more inclined to believe your issue would be resolved by adjusting the reel's settings moreso than changing the mono backing.