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Sure you can fill the 4500 from a scuba tank.
You'll only be able to fill it to 3000psi, not as many shots.
The down side, most 4500 are much smaller.
Here is a link Looper gave me just a few days ago, hope it answers your questions.
http://scubatoys.com/paintball/scubafills2.asp
Sorry - right site wrong page.
Try this one:
http://www.scubatoys.com/paintball/paintballshots.asp