Markin and Weber in Kelowna does good work. Not fast or cheap.
No 4 barrels aren't interchangeable as is. They need to be chucked in a lathe and have the shoulder set back to index properly, then they may need to have the chamber cleaned up for proper headspace with the bolt you have.
It also takes a special set of tools to properly hold the receiver as they're quite soft and will easily deform if to much force is used or an improperly fitted wrench is used.
There is no way to install that barrel with an internal action wrench and there is only one useable flat and it's on the bottom of the action, to take up most of the torque.
A properly machined outside of the ring, action wrench, with a slot cut into the flat for the King Screw lug on the bottom is a must to get the right torque, without deforming the action.
I've seen several No4 actions that were damaged because some nimrod decided to use a Crescent Wrench on the side flats, that had to be discarded.
Markin and Weber have the proper tooling on hand and if they don't, will make it up for future use on other actions.
Good folks doing good work.