No4 bolt heads are marked up :
0.1.2.3
The sizes SHOULD be slight incremental increases with 0 as the smallest.
Some time ago I investigated rumours that the size (number) was almost irrelevant to the actual bolt head size.
I (and a few friends) measured over 100 bolt heads and found that in fact you could get a number 1 bolt head smaller than a No2 bolt head, or a No2 bolt head larger than a No3 bolt head.
The actual results were :
Size / Number 0 went from 0.620" - 0.631"
Size / Number 1 went from 0.622" - 0.635"
Size / Number 2 went from 0.630" - 0.640"
Size / Number 3 went from 0.632" - 0.640"
If you need a new bolt head specify the dimension needed - not the 'next size up' - you could end up worse than when you started.