The entire unit must move with same movement-characteristic.
Since Flamers cant Fly, they all have to move by ground.
So "walk" with M4 or march with M8 is the only options.
I gave this a good thought when the book first came out and came up with this solution:
Take two units of Flamers and one HoT with wings.
Join HoT in first unit.
Firstly march with second unit, then Fly with HoT to second unit, last march the first unit.
Probably common knowledge for most Daemon-players.
Most opponents on the other hand said: "Cheeese!"