If the G4's ports are auto-MDX, then you don't need a crossover cable. It won't hurt anything if the port isn't (and an old G4 might not be, I don't remember), but it won't work either. You might try getting a dumb hub to chain them together. I prefer hubs for my internal network infrastructure because they just transmit and don't care about the actual packets.
EtherTalk doesn't use IP (don't confuse it with AFP over TCP: this is straight AppleTalk), which is the advantage here. If EtherTalk works, then the problem is at the TCP/IP level. Make sure your PowerBook can speak original EtherTalk (preferably in OS 9 or lower). You can test this simply by trying to share files. If that works and the computers show up in the Chooser, then you know that the hardware is not to blame.
Otherwise, you can use ICMP (ping and traceroute), which is not TCP, though it is IP. However, this presupposes you have an IP address, so I'd try AppleTalk first. If you give yourself a static IP, try using one of the Mac ping utilities on the 520 to see if you can reach other hosts on the same subnet.
EtherTalk doesn't use IP (don't confuse it with AFP over TCP: this is straight AppleTalk), which is the advantage here. If EtherTalk works, then the problem is at the TCP/IP level. Make sure your PowerBook can speak original EtherTalk (preferably in OS 9 or lower). You can test this simply by trying to share files. If that works and the computers show up in the Chooser, then you know that the hardware is not to blame.
Otherwise, you can use ICMP (ping and traceroute), which is not TCP, though it is IP. However, this presupposes you have an IP address, so I'd try AppleTalk first. If you give yourself a static IP, try using one of the Mac ping utilities on the 520 to see if you can reach other hosts on the same subnet.