Mudguards at bicycles are usual fixed with M5 screws. M4 for a MORGAN would be a little bit to small.
A few years ago I customized a friends Harley. I fixed the carrier for the rearlight and the numberplate with three M5 screws and selfsecuring nuts.
The Harley vibrations undo the screws in a VERY short time.
So I think more on the M5 than M4.
Greets René
