Maybe I am looking at the wrong spot, but if I go to gog.com, profile, then to the library, select SS4 all I can see is 11 parts of the SS4 installer, 1.01 -> 1.02 patch and 6mb Deluxe edition DLC that claims that it is going to install something but I do not really see what it installs and I do not see anything that would look like soundtrack

It's not there yet. I don't know what he's talking about.

I have it in my account (see attached screenshot).

The library entry for Serious Sam 4 seems to be completely bugged on GOGs side. I had to do an account refresh for another problem with my game library yesterday (merging Lonely Planet OST with main game) and that might have helped fix the missing SS4 soundtrack.

Try doing an account refresh by clicking on the following link, wait a minute or two and then reload your library.

https://www.gog.com/account/refresh

Damn. Thanks. It worked. Sorry if my response came off as overly aggressive :P

Glad it worked for you too.
GOG still needs to fix the underlying problem though, because the extras still aren't available via Galaxy, even after an account refresh.

That worked, thanks!