I've been thinking of getting one. From what everyone says, they are excellent rifles and quite accurate. As for parts, they could be hard to come by, but Marstar looks after what they sell so you should be able to source parts. As for Original No.4 parts swapping over, I have no doubt they will no fit, it may go by the No,4 name, buts is a completely different rifle other then the action style.
My opinion is, go for it. The only compliant I hear with them is they are really heavy, but if your going to do just bench rest or prone shooting, your good to go as a heavy rifle is desired for a target rifle. Also, they are pre drilled and tapped for a scope mount, which Marstar sells so scoping these is a easy as paying a few extra bucks for the mount.