Interesting. Do the No.4 bolt heads interchange with No.1 boltheads, because I jockeyed headspace on a couple No.1's back in the day when those bolt heads were floating around the gun shows a dime-a-dozen, and I've never owned a No.4 in my life.
No, the No 1 boltheads are quite different from the No 4. No 1 boltheads are interchangeable with each other, and can vary slightly in dimensions due to engineering tolerances, and so you can go through a box of them and try to find one which will give you the headspace you want, but they weren't intended to be swapped around like that.
With the No 4, one of the production engineering innovations was to make boltheads in specific lengths. The idea was that this would allow the assembly of the barrel to the action to be achieved by a semi-skilled worker, simply screwing the barrel into the receiver and then using different sized boltheads to get the headspace into spec.
This was in contrast to most other rifles, including the SMLE, where you needed a fitter to adjust headspace when fitting up the barrel, with a fixed-length bolt assembly.
One furphy, which seems mainly confined to the US, is that the different-length bolt heads were intended to be used to adjust headspace of rifles after they've seen a bit of use, on the supposed basis that this needs doing every now and again with Lee Enfields. This is not correct. It is true that LE's, especially wartime production, can tend to have fairly generous chambers, and sometimes generous headspace, but that is how they were built - mainly to make sure they got to the troops in sufficient numbers and worked when they got there, even with dirty ammunition, and with no thought to use as a sporter or with reloads.