Dudes, forgive my ignorance I'm dyslexic it seems that I have a custom adaptor on my hub, it's 4 stud and they measure 4.5" diagonally (stud centre to centre) and 5" (outside stud to outside).

What's my pcd?

It's an old datsun, the pcd is supposed to be 114.3, which is 4.5"

The way you suggested measuring I've been told isn't right thought, it is supposed to be measured diagonally, centre to centre

Stud centre to stud centre (diagonally opposites) on a 4 stud car will give you the PCD. So, if it measures 4.5" then it's right. Odd number of studs (3 and 5 stud basically) become a bit more difficult to ascertain the PCD.