As far as the gun is concerned it just takes the air so it does not matter if the cylinder is 3000psi or 4500psi
A standard steelie at 3000psi is in fact an aluminium cylinder.
This will work for most people, and will have a flat bottom
Anything with a flat base is at maximum 3000psi.
A cylinder with a curved base is probably a fibrewrapped 4500psi cylinder.
A fibrewrapped cylinder is a thinner aluminium, but fibre wrapped for reinforcement which gives more strength and can allow higher pressure.
An aluminium cylinder is heavier than the same sized fibre cylinder. But not the vast difference in weight that some will say. Balance is more relevant. A 'standard' fibrewrap is bigger than a 'standard' aluminium, which means they are closer in weight.
An ultralight fibre cylinder does have a real weight difference.
If in doubt buy a standard aluminium cylinder at about £30. For air capacity and lifetime it is the best VFM
This will last 5 years minimum, and if the site accepts it, aluminiums are valid for 10 years. You can then have it hydro tested, but its probably not worht the time and money.
A fibre cylinder will cost around £150, or more for an ultralight or if you want a specific size, shape, regulator etc.
Then it expires at 5 years, and can be retested until it is 15 years old. It will cost about £200 for 15 years of life if you get it tested without having to post it.
Most of the time there are only fills up to 3000psi so you usualy won't get the extra air capacity to 4500psi.
The real reason to buy fibre is for balance and ergonomics. The right size and shape to fit you.
With a round base (which is now available on some 3000psi cylinders) you can roll the cylinder in your shoulder giving an ideal stock for paintball.