Hi, Joj. I hope that you will get an official response from the folks at MLO soon but in the meantime here's my take as someone who has been using MLO for a while.
I would expect an activation code for 4.3.3 to work for 4 dot anything. When version 5 comes out, I would expect people with a 4.x license to pay an upgrade fee for a 5.x, but i would expect it to be a substantial discount from the price of a new license for 5.x
We are just beginning to hear the first rumbling rumours of work on 5.x so i would assume (no guarantees) that it will be at least a couple of years before support of 4.x ends.
-Dwight
MLO Betazoid on Android SGN4