I have used this device to digitize VHS tapes into Macs with great success. It's affordable and easy to use.
https://www.elgato.com/en/video-capture
However, the problem here is that it requires Mac OS X 10.6.8, and even though that system version is 10 years old, it is still too new for the two Macs you have. Your might try to hunt down a different VHS digitizer that is compatible with your older Macs. But that might be a challenge because those Macs are so old.
A more practical option is to find a newer Mac that can use the device I linked to, or something similar. It wouldn't have to be a very new Mac, that type of device works with Macs several years old. It's just that the Macs you listed are ancient. The G4 is between 15 and 20 years old. The PowerBook 1400 is at least 22 years old!
You may be able to find an old VHS digitizing card that can be installed in one of the expansion slots of the PowerMac G4. But I can't even remember what brands those were. It would take some effort to locate one that is specifically compatible with the G4 slots, since the standard for those has changed several times in the last 20 years.