SH4 is really nothing more than SH3 part 2: the American U-boats edition. Most if not all the file structure from SH3 is still present in game. Once you start digging into the modding of it you really start to see it more. Take for example the differences in the propulsion setup between a fleet boat and a uboat. Uboats had 2 diesels and 2 motors on a direct dive system, meaning you had to clutch in the motors to charge batteries or for underwater propulsion. Fleet boats have a 4 diesel and 4 motor setup. The diesel ran generators only while the actual propulsion was provided solely by the 4 high speed motors. Regardless of what boat you have in game, you only have 2 diesels and 2 motors.