Hope it saves some time to fellow subsimmers. Go check on (and donate, though not obliged), in particular this thread, by THEBERBSTER, which tells you everything you might want to know: You can go a bit farther than me in terms of realism (play with TDW real navigation) but then it-s impossible with the Steam version (need to use an incompatible TDW patcher.) SH4 remains IMHO the historically richest and most dynamic and varied campaign there is (TMO, OM, etc.), but SH5 - modded with Open Horizons and more - is a significantly improved and immersive experience, with even more realism as a simulation if you want. There are fantastic modders out there (DarkWraight, Sober, and many others) who transform the somehow unfinished stock version into an amazing subsimmer with big advantages over SH4 (graphics, crew management, overall immersion). I wanted to play with the most realistic (physics, historical accuracy, manual targeting) and graphically cool mods.
Longtime SH4 player I have spent many MANY hours on SUBSIM finding a satisfying modlist that didnt CTD too often.
SH5 is simply great (as long as you mod it).