Hello, I, for one found the music of Tropico 4 unfitting the theme, because Tropico always struck me as a comedic, light-hearted game and the music was too serious.
This is a simple tut, I have tried on the Demo, but should work in the full version.
Go to: Tropico 4/Packs/boot and back up the file music.hpk (in case something bad happens, or you want the old music back).
Now, go the Tropico 3/Packs/boot and copy Music.hpk from there
Paste it back in the same directory, within the Tropico 4 folder and allow it to over-write.
Now join the game and Enjoy Tropico 3 music
And yes, it should work the other way around (Trop 4 music in 3)