IF you want to play Silent Hunter IV you must have Silent Hunter IV gold edition (gold edition includes U-boat expansion pack and 1.5 version of a game which MAJORITY of mods work with). Best option is to get the game through amazon download
https://www.amazon.com/Silent-Hunter.../dp/B008X73CE8
Silent Hunter III is still a great game, a lot of mods to choose. I would recommend getting it