Yes, Hiro 4 is based on M4 (similar to how Aiko 4 is a morph for V4 but also a stand alone figure because the head and eyes are so different). You can use M4 clothing on Hiro 4, but it will need the HIro 4 body morphs, or Hiro 4 tailoring magnets for the correct fit.

M4 is not my style and neither is V4, so to me adding Aiko 4 and Hiro 4 to V4 and M4 makes those figures more appealing. And there is the added bonus they can share clothes. 

It's possible that an improved M4 will get released along with Hiro 4. M4's been around long enough now that problems with him have been found. For instance, it looks like H4 has better hands. I don't know if DAZ fixed other issues like the feet bend.