If the software and hardware you're using calls for using the native ports, then you will want to plug the device directly onto one of the ports on the motherboard of your system. However, there were several USB cards that work well in Power Macintosh G4s.
Most USB 2.0 cards will also provide 2.0 speeds when booted in OS X and 1.1 speeds when booted in OS 9. Unfortunately, this is a limitation that will never be overcome, unless somebody writes new drivers. One way to improve external storage performance of a Power Macintosh G4 is to use Firewire, another is to use eSATA.
On a personal note, I recommend against the blue five-port Belkin USB 2.0 card. It advertised Mac compatibility, but in my blue-white/Yikes system, it was identified as being the cause for the system not booting up, so I"d have to pull the card every week or so and wait until the system would boot up again. I eventually either moved that card to a PC or to my Power Macintosh 7300. I had a friend with an MDD Power Macintosh G4 who also had problems with a similar Belkin USB 2.0 card.
Most USB 2.0 cards will also provide 2.0 speeds when booted in OS X and 1.1 speeds when booted in OS 9. Unfortunately, this is a limitation that will never be overcome, unless somebody writes new drivers. One way to improve external storage performance of a Power Macintosh G4 is to use Firewire, another is to use eSATA.
On a personal note, I recommend against the blue five-port Belkin USB 2.0 card. It advertised Mac compatibility, but in my blue-white/Yikes system, it was identified as being the cause for the system not booting up, so I"d have to pull the card every week or so and wait until the system would boot up again. I eventually either moved that card to a PC or to my Power Macintosh 7300. I had a friend with an MDD Power Macintosh G4 who also had problems with a similar Belkin USB 2.0 card.