John, I think it depends on how you want the bike to feel. The 4.00 rear tyre was big for its time and I suspect it was run at low pressure for the softer ride that would give. I think this is also the reason for the 'wallows at speed' comments about square fours in old road tests. 30psi is a good starting point with new tyres.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.