OK, then either:
pilot 3.3 mm for an M4 x 0.7 (coarse thread)
pilot 3.5 mm for an M4 x 0.5 (fine thread)

In general I would use coarse thread, especially into aluminium. Get a countersink to clean up the hole and get snug fit of the rail onto the plate - snail countersinks are great.

Also, thread lock (the type you can undo) the screw into the hole as the spindle vibration can undo them.