It's not possible unless you rebuild each mission manually yourself. EM3 and EM4 are two seperate games and no add-ons of eachother. EM3 coding is different. For example, the map formats are .e3m while in EM4 they're .e4m. Same thing for the prototype files (.e3p and .e4p) and you cannot convert them. Besides that, both games have different objects. You would need to copy hundreds of objects.