Welcome to SubSim Voxman!
As was pointed out by a couple others, I wouldn't upgrade using a Download. I did years ago with a site called Direct2Drive. After about a year or so of time.....if you wanted to re download from the site again, you had to call tech support to have them re issue the code for accessibility. Some time later, D2D was sold to another company and I just didn't want to get into the hassle I was going to have with proving I purchased the download rights from years earlier to a third party.
Today, a disk copy can be had through several sites. Look for the "Gold Edition". It has the complete game updated to version 1.5 (which, as you have learned, has the U-Boats Add-on included). InetVideo is the site I purchased mine some years ago. The DVD of
Silent Hunter IV Gold Edition (click the name) can be purchased for under 15.00 dollars U.S. Took all of 4 days to arrive, coming from the Montreal Canada UbiSoft head quarters.
If you plan to use some of the mods found here, you will need to do just a couple of things to keep your game running smoothly. JSGME (JonesSoft Game Mod Enabler) is an app that will add mods to your game (most mods are compatible with JSGME). One thing that's critical to do before ever trying a mod......Allow JSGME to "Take a Snapshot" of your freshly installed game files, so you can "Compare" the game files at a future time for possible corruption (due to mod activation/de activation). This feature is the best way of checking your main game files for any possible changes.
Another thing is to only add mods that are compatible to the version of the game you have. If you have version 1.5.....only use mods that are made for a version 1.5 game.
Happy Hunting.