Here's an easy way to tell if your 40s steel is a 4" or 4-1/2".
If you lay a straight edge across the centre of the wheel and up against the outer rim, On the 4" wheel the end will sit just outside the rim. On the 4-1/2" wheel the end will sit just inside the edge of the rim.
4" wheel:
4-1/2" wheel:
A handy little tip especially when the tyres are still on!
Mart.