Solved V4 just now. Very neat puzzle, although quite dastardly as well, especially with all the number crunching I had to work out. My solution ended up not using the Combo Brown+Pure or the Brown Exact -1. Not sure if I got an alt or if those were meant as some form of anti-cheese. If the latter though, then I might be able to implement a cleaner version of it into the game using some new mechanics from Part 2.
For posterity, my solution was (rot13) Terra Erq Checyr Oyhr (Plna Oynfg) Oynpx (Cher +1) Juvgr Tyvgpu Oebja Cvax Benatr