As far as I understand, actual mods for MW4/mercs require practically taking apart the program and modifying it to support specifically made new assets and then putting the program back together in a way so that the program understands how to run it. Might have worded it incorrectly, but that's what sometimes happens when I paraphrase an idea from my thoughts. Basically the game is not meant to support any sort of mods at all and getting it to work with modifications requires some extremely low level tinkering with the game itself, if only compared to other games that have mods.
Mapping, on the other hand, is obviously fully supported, otherwise there wouldn't be an official map editor (though the modified versions of the editor are better, kinda like Atan's versions of D3Edit for Descent 3 being better than the official versions).