The contact completion in the mail composer window (To:-field + <Tab>) suggests contacts only based on the email address, not on the contact name. This is based on a vCard address book (and was different with 3.x, AFAIR).
Working fine here, matching name or email address. Do you mean your address book only has vCard entries?
Yes, exclusively vCard
I set up such an address book, exclusively vCard, and it works just fine.
Issue solved - completion scrollbox in 4.0.0 is much smaller than the 3.18.0 one and the scrollbar is barely visible, but all contacts can be reached by scrolling.
see a related bug #4491