Thats exactly it. All bottles come in the same basic sizes, 68ci, 88ci, etc. Thats a measure of volume in cubic inches, but with a 4500psi, which is a measure of pressure in pounds per square inch, tank instead of a 3000psi, there is literally more air compressed into the same area, wich yields higher pressure. Functionally, it works better on a higher pressure marker, and of course a 4500psi gives more shots per ci than a 3000psi.
Interesting note, you find 3000psi tanks made of steel (these are REAL heavy), but I don't believe anyone makes a 4500psi made of steel, only fiber wraps. This is because the walls on a steel 4500psi tank would be nearly 3 inches thick.