Yes, they are interchangable. Of course you need to remove the pump from the bracket and install it on the 4cyl bracket which requires a special tool. You can usually rent them from your local O'Riely or equivalent.
There has been some discussion about you can use a 4cyl pump on a 5.0 rack or a 5.0 pump on a 4cyl rack and some poeple say that you cant use a 4cyl pump with a 5.0 rack because it doesnt produce enough pressure and will lock up the steering or something. I call bull**** on that one and I have personally decided that it doesnt matter. I have done it both ways and never had a problem. I have 2 cars right now that have 5.0 racks and 4cyl pumps and work flawlessly.