4500psi scuba tanks are rare, and most scuba shops won't fill to 4500. Scuba tanks are usually 3000PSI, so you can fill your HPA tank up to 3000 PSI from it on the first fill. Each subsequent fill will be less and less pressure until you go "boy this sucks" and go get the scuba filled again.
Standard scuba bottles are 80 CF not 50 like you're looking at. Do not try to buy used scuba gear unless you know what you're looking for. I highly recommend that you go to a scuba shop and ask them if they have any used tanks that they will sell to you (and fill.) Typically scuba shops offer "fill cards" for a discounted price. At the shop I go to, fills average out to ~$4/fill for an 80CF 3000psi cylinder.
ill just look for 80cf 3000psi