May be it also has to do with the quality of game playing. The graphics might be way better in sh4 (i own both SH4 and SHIII), yet I relate so much more to SHIII. For a starter a U-BOOT is much better looking than any truck looking submarines in SH4, then there is something almost mystic that captures the imagination about U-Boot, that does not exist on SH4. Finally, you know the outcome of the WWII, so why would you enjoy playing a game that will end up in victory whatever your outcome, when playing U-Boot you know you will loose, yet you want to give a hard time to the AI, the challenge is bigger. May be those are part of the constant interest for SHIII instead of SH4
