Having myself experience with such a conversion

I can well understand the size of the task you are planning and wish you best of luck. Yet I don't see any clear advantages in SH4 when compared with SH3 which justify the huge amount of work IMO. I mean, aircraft work well at high compression rates and graphics are a bit more fancy -but SH3 at high resolutions with AA and mods like GWX looks really well already- but that's more or less everything

None of the special missions of the US subs were assigned to german U-Boats normally, and you still lack wolfpacks and milk cows

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Anyway, I don't want to hijack your thread, this is not a discussion about if it is worth doing the conversion