wget to an IPv4 server from IPv6-only VPS

IPv6 and IPv4 are different protocols. If you only have IPv6 address in Internet space, you cannot connect directly to IPv4 only services. It's really short answer - "no, you can't".

TunnelBroker and other free services offers only tunnels in other directions, i.e. from IPv4 to IPv6 and not vice-versa as far as I know. You can set up your own tunnel on another host running dual-stack (both IPv4 and IPv6) or try to find some commercial service for that purpose, but this is out of scope of Server Fault and I doubt that this is what you need.

For downloading a file you can use other PC and just transfer it to your IPv6 box over SSH.