Jeeves,
The 3K/4.5K thing isn't a strict 50% more as that implies that the whole of the tank is "usable" where as in reality the last say 500 psi is too low.
However, the other consideration is fills. In the UKyou can only really get a 3000 psi fills except at Millenniums. Some dive shops will fill 300bar bottle so they give about 4K but that's not a given and most sites like sparklies with the bulk bottles, they only give about 3K tops anyway.
So while 4.5K is preffered, it's by no means essential.
The 3K/4.5K thing isn't a strict 50% more as that implies that the whole of the tank is "usable" where as in reality the last say 500 psi is too low.
However, the other consideration is fills. In the UKyou can only really get a 3000 psi fills except at Millenniums. Some dive shops will fill 300bar bottle so they give about 4K but that's not a given and most sites like sparklies with the bulk bottles, they only give about 3K tops anyway.
So while 4.5K is preffered, it's by no means essential.