3000psi fibres are possible, but you can also find cylinders where someone has changed the regulator. Never make assumptions based on what the cylider looks like.
Check the full markings of the cylinder, the regulator and the burst disk
To be a 4500psi cylinder it should have two pressure ratings in the markings
e.g. I have the following markings on cylinders
48cu alumniums:
Labels show 3000psi and stamps show PW207 PH311BAR (3002psi / 4510psi)
13ci aluminium:
Labeled 3000psi is stamped PW205PH310BAR (2973psi / 4496psi)
Fibres:
PW310PH466BAR is a cylinder with a working pressure of 310BAR/4496PSI and maximum test pressure of 466BAR/6758PSI
PW345PH517BAR is a cylinder with a working pressure of 345BAR/5003PSI and maximum test pressure of 517BAR/7498PSI
Burst disks:
My 3000psi aluminium have 1.8k & 5k burst disks, on the regulators except the 13ci which only has a 4.5k disk
My fibres have 1.8k and 7.5k burst disk
The lower burst disk is the protect you & your marker from the regulator output, the higher to protect the regulator, you & your marker from an excessive input. You must not decide to overfill a 3000psi cylinder just because the safety rating & burst disk are higher.
You can only fill to the lowest common denominator out of your cylinder working pressure, regulator and the high pressure side of the burst disks