Rumour has it that the DCRA target rifles had some #4 bol theads made prior to changing to 7.62NATO in the mid to late 50's. A #4 bolt head is not a standard bolt head.
In any case, like John says, if the headspace is ok, don't worry about it. However, if the bolt is closing on a No-Go, but not a Field, the headspace is on the high end of tolerance. Still not going to be an issue, but if it goes bad, the barrel will have to be removed and the threads set back by a smithy.