Do I need to downgrade GTA IV to use mods?

not always these days. a lot of current mods are made for the Complete Edition with FusionFix installed, and that covers most of what people used to downgrade for. you only really need to downgrade for older script mods and big packs that were built for 1.0.7.0 / 1.0.8.0 and never updated. read each mod's requirements before deciding. if you do downgrade, back up the whole game folder first, because a Steam file verify will put the new exe back.