True, the H3 and H4 (and I think the H2F) can't take 3rd-party backs (nor film backs, which is annoying to some pros i've heard).
Except for the new H4X, can take any leafmiyaone back like the H1 and H2. Maybe the H4X is Hassy's way of saying, "our backs aren't that good, please don't swap systems, you can use someone else's backs just please keep buying our glass"?
Quote from Hasselblad's site. http://www.hasselblad.com/H4x
"Give us your old H1/H2/H2F body and we’ll give you a brand new H4X, the super-charged offspring of the H1/H2, for just €3,995 excl. VAT. This special H4X offer is only available to H1/ H2/H2F owners and is not valid without a trade-in of a Hasselblad H1 or H2/H2F camera body."
Only available buy trading in your old H1/H2/H2F body. Sounds more like they don't want people with Leaf, Phase One, Sinar or Jenoptic, etc backs replacing their H1/H2/H2F bodies with Mamiya or Phase One bodies.