When using braid on a spinning reel I use a mono backing to help with the slipping. Recently I bought some 30# braid and tried using about 50 yds of 4 # mono backing. I notice that I am getting a lot of wind knots with this. I tied them using a uni to uni knot. Should I increase my mono backing #? Thanks in advance.

No it's not the backing, what you are using is good, I tend to go w/ 6 lb mono.  What's causing the wind knots are the line is twisting.  Either you spooled the braid w/ a twist in the line & didn't know it, or you used a lure that twisted the braid.  

I get wind knots especially after using certain lures, like in-line spinners.  To avoid wind knots, I use a swivel & that seems to help.  I would recommend that if you have access to a boat is to unspool the line in the water while the boat is moving, up to the end of the mono on the spool.  Let all the line drag in the water for a while & then wind it back up w/ some finger pressure on the line.  This will help get any twists out of your mono & braid.

I don't think your wind knots have anything to do with what kind of backing you're using. Neither your casting nor fish-fighting should involve the mono backing; if it did, that would mean you have too little backing. I always figured that wind knots are one of the downsides of braid that we have to live with if we want to enjoy its advantages.