http://www.evcom.net/~n4mwd/cscuba.htm
these are pictures of why you may not want to go higher than your 4500 psi tanks. There is a thread on buying scuba tanks and going higher somewhere here. But really what you are carrying next to your body is a little bomb. Come fields around here are using air compressors to fill scuba tanks, then bleeding off the air for quick fills. This is cool, but be careful and let someone who is certified to fill the scuba equipment up do it.
I personally dropped a 6' co2 tank off a dock while loading it onto a truck. The nipple broke off and shot the tank through two walls of the warehouse. I can only imagine what would happen under 4500 pounds of pressure.