No.4 MkI can be fired form half cock

[Towarzysz_Beagle]

If you have a "spare" sear, swap it in. If the "long arm" of the original sear is distorted, (bent), it may well be engaging the very end of the half-cock notch and this not locking up the bolt and trigger movement.

Swapping in an "unmolested" one is pretty easy, after removing fore-end first. You then have to attach the trigger-guard assembly in its correct location (sans wood, of course) and see what is happening.

Please keep us posted.

Late update, but you were spot on mate.

I took the forend off and attached the trigger guard and bushing. The sear is being caught by the half cock but not making it all the way into the notch.

The interesting part is that the original sear does not seem bent or damaged in any way. It engages properly with the bent, and disengages consistently when fired. It also engages correctly with the safety.

The rifle does have a non-matching bolt. Which bears well on both recoil lugs (first thing I checked on the rifle). I'm guessing this issue with the half cock is a matter of fit, and I am unsure how to correct it.