You already know that filling from a 6000 psi tank into a 3000 psi gun is dangerous if not done carefully.... Most in that situation would either get the appropriate regulator (about $400).... or a "slow-fill" valve that has a restrictor upstream of the valve and gauge so that you have very fine control over the fill rate, and just watch the gauge like a hawk.... Most 4500 psi fill setups use the latter....

Bob