It can be done, you may need to engineer a tool that fits into the oil channel in the crank to help remove the upper main bearing halves. Be sure to use lube on the new rms, it will tear otherwise. However I don't believe this is worth the effort. Let me explain.
If the knock is indeed a bearing issue, crank journal scoring is likely. To correct this scoring the crank journals should be resurfaced. I've seen two 4.0s with knocks, one was a wrist pin knock, the other was a broken piston skirt. I have not seen a 4.0 knock with a bearing issue, I'm by no means a 4.0 expert either. I'd highly recommend pulling the motor, and doing a full rebuild. It's the only way to be sure you have completely resolved the issue.
My .02$
Jeremy