With OSX 10.3 (Panther) you are restricted to LimeWire 4.12.15. Unfortunately LW 4.18 will not connect (upload to) with any version older than 4.14. However still plenty of old version users around. But eventually this number will decrease over time. The reason for Panther's limitation is the version of Java which is version 1.4.x. Tiger (OSX 10.4) at least uses Java 1.5. And Leopard (OSX 10.5) uses Java 1.5 & 1.6. The new LW 5 when it is finally released will be restricted to Java 1.6 users. Again, earlier users of OSX will be affected.

Use Basic in meantime perhaps. If it is possible, upgrade your OSX.
My old G4 (733 MHz) uses Tiger OSX 10.4 without issues. Another G4 here (400 MHz) also uses Tiger but is incredibly slow. lol