I'm sorry if this is a silly questiuon, but what version of the Settlers 4 is the one at GOG? Should I apply the 2.50.1516 patch?
General word of advice: do not, under any circumstance, manually apply official retail patches to games from digital distribuition services like Steam and GOG (with the exception of GOG-specific patches that you can see in your library, they are adapted to GOG's releases). There's a chance that the patching might succeed, but if it does, it will in all cases bring back the CD check and/or other DRM that the game used to have, and possibly cause further errors that can only be effectively solved by reinstalling the entire game. Steam and GOG always have the latest versions of games (that were available in English-speaking territories, at the very least).