This topic has come up before but I did not find the actual solution in the forum. But I managed to figure it out myself so hopefully somebody will find this helpful.
Problem: at some point in time my P4 starting forgetting the last screen layout and always reverted to 2 decks vertical (which is the last thing I want to see on my screen) after every reboot. Upgrade of the firmware did not help. Some old thread suggested factory reset, did that and reapplied my settings, no good.
Solution: Do NOT turn off Deck 3/4 in the settings (which is exactly what I had done at some point)! Last layout is remembered only if all 4 decks are enabled, ie you can switch between four different views with shift-view. In other words - if you want your Prime4 to remember that you want to see the 2 deck horizontal view as a default you HAVE to have all four decks enabled.
Have to say that it’s one of the weirdest software glitches I have recently seen but I am glad I found the solution. If this helps anyone then you are welcome.