It is not going to be easy to find and purchase expansion pack 4. I think that is a dead end. IMHO
Expansion Pk 4 is so old, it may not be worth it. Where are all the Mig Aircraft on the Swaps?
The only Mig-15 I have is a prototype hand launched one that I don't believe was ever finished. CS isn't really there and the collision mesh is missing. I had to go back and open the old 7.5 to find it.
Some Mig content is on the swaps, you just have to be creative to find it. Still, there is a problem with the search engine. Remember that finding an AV (Aircraft Variant) or CS (Color Scheme) file will lead you back to the EA (Entire Aircraft) file.