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My opinion is that anyone competent to rebarrel M1A/M14s would have no trouble with No.4 barrels. Both have inclined locking surfaces, variable receiver face lengths (the commercial M1a are almost certainly worse than any variation in No.4s), indexed sight attachment points, and bolts that have their own variables. The good thing is that Critereon is quite familiar with the M14 headaches, so it's unlikely they'll be much surprised by No.4s.
Good point on the M14/ M1A issue.
The difference is that M-14s are made from 8620 steel and have a "square' section breech thread, whereas No4s (at least Mk3 and 4 type receivers) are made from, and I quote from the drawing: B.S. 970 En19R or B.S. 970 En 21R and have a Whitworth form thread.
See: Drawing D5(E) 0355/231.
8620 is a very interesting alloy steel that can be heat treated to provide a very hard skin in a very tough interior structure with minimal distortion.
B.S. 970 is a British steel standard first issued in 1941. En19 series are 1% Chrome/Moly with several variations. This is quite a change from the almost purely carbon alloys of the SMLE series.
In the last few years, I have had the opportunity to study the barrel drawings for M-1s, M-14s and No4s. I have also re-barreled several No4s and M-1As from blanks. Back when Australia still had something of a sense of humour about such things, I found the "easiest" way to do M-14 barrels on a one-off basis, was to:
Rough profile.
Thread and counterbore the breech end.
Rough chamber
Profile
Cut muzzle threads and crown
Fit a muzzle "nut" with centre cone and profile barrel.
Fit to receiver with final torque
Cut final chamber with a "drop" reamer
Mill feed ramps
Mill keyways and slots as required.
Drill gas port
This is fairly time-consuming as you would imagine. Doing them in batches cuts costs. Having a barrel that is "80%+" finished and only needs minimal "tweaking" by the gunsmith is the ideal.
HOWEVER, said gunsmith needs to be "Lee Enfield savvy" with regards to such nasties as receiver wear, twist and stretch for starters. "Tired" bolt bodies and bolt head are supplementary curses. With a No4, it is a good thing if the front sight mounting hugs index correctly as well.
If someone wanted to make a production batch of either M-14 or No4 barrels, they would either need to do some serious reverse engineering or get their sticky paws on the "charts".
In the case of No4s, the person doing the final fitting definitely needs Lee Enfield "smarts" as well.