Ahoy, Arlo...
Yeah,
Steam has 2 different versions of SH4... 1 without Uboat Missions tag attached to Silent Hunter IV Wolves of the Pacific... the other (which is the v1.5 one) is tagged as Silent Hunter IV Wolves of the Pacific: Uboat Missions.
The latter version, is the 1 you want to download... & install.
M. M.