Hi Alan and welcome to the forum.
Look up “Marks model bits”. He will make a a 4 inch propshaft for you . Assuming you want a ‘standard’ 4mm shaft. The cheap ‘radio active’ 2mm shafts are very easy to cutdown if a smaller shaft size could be accommodated, as the bushes push out easily enabling the outer tube to be shortened.
What is the shaft intended for?
Ashley