Can he wear M3/V4/V4M clothes?
Short answer: NO. To convert clothing to M4 you can use a Clothing Conversion program like Wardrobe Wizard 2 or CrossDresser. Long answer: Boots and footwear for M3 will often fit M4 with some tweaking of the footwear’s morphs (mostly to enlarge the calves). Some smaller V4 clothing items can also be conformed to M4 and tweaked with morphs (the V4 male morph is particularly useful for this). Props can be scaled and re-parented to M4 and you can also tweak some M3 clothes by hand (especially pants) to fit M4 using this tutorial.