Will it help me get a better connection than trying to connect 2 level 3 boards through wds or other means? Or will I not notice much difference? Can I upgrade 1 board to level 4 and get better results?
if you didn’t face any level 3 restrictions - you do not need level 4
Most people upgrade to level 4 when they want to run a RB411 as an AP. (PtMP)
You can buy them from a dealer, a distributor, or directly from Mikrotik.
But don’t waste your money.
It won’t improve the wireless performance.
Tom
if all you want is to make wireless hopw with both ends RB411 - level 3 license should be enough, because level 3 allows bridge mode that allows 1 client, and that is all you need.
thought level 4 does not cost that much. 
or choose RB that already have level 4 installed from factory. and use one of your RBs elsewhere.