dont see why you need twin overflows on a 4 foot tank. too small - unless you are only doing this for redundancy. but if the first blocks up, you'll have more troubles than this. imo, twin overflows only need doing in 6ft and larger tanks UNLESS your water turnover is MASSIVE.
however, you should be able to reduce flow through one overflow without it overflowing. if you put in a durso standpipe, you could adjust the water level in the overflows that way and adjust the flow rate my using a ball valve on the piping outside the tank. too finicky and open for error though IMO!
I even took a photo for you
I dont bother adjusting mine as I want it fast as possible! If it were on your setup, you would simply turn the blue tap to adjust flow rate.