From download link, at top this page, according to your device and operating system.
I Have downloaded Skype Version 18.104.22.1680 but it says " "You can't open the application "Skype" because it is not supported on this architecture, and has a crossed out Icon in the Finder
I have Powerbook G4, syst 10.5.8 PowerPC (not Intel), 1.33mhz, 2 gig ram, 320 gig HD,
The old software worked OK, can I still use Skype on my PB G4?
Version 5.x is Intel only whereas your G4 is running a PowerPC processor.
Skype 22.214.171.1246 version should work fine as it's UB (Universal Binary).
Downloadable from http://mac.oldapps.com/skype.php
You are welcome.
Enjoy Skypeing !