Version 1.3 of the game did have Save issues, similar to SH3's, as well as v1.2 and 1.1 of SH4... Version 1.4 did fix most of those issues, but you cannot apply the Ubisoft v1.4 to a Steam v1.3. The Steam game will not take it. With Steam games, you have to stay "Steam", which leaves you with the Uboat Missions add-on from them. If you have Steam v1.5 on your other computer, you can also load it onto this v1.3 laptop you are dealing with, and Steam will handle the licensing just fine. In other words, when you start the game on one of the computers, the other computer cannot then start the game. If you want to apply mods to the v1.5 of the game, you'll have to
move it into a different Steam "Library" folder that you make outside of the Windows Program Files folders. SH4 v1.5 does "dynamic loading" - only loading 3D "assets" that are within your immediate area, which lessens the strain on older computers.