You have several options. All will work.
Wiping everything / departition / install Tiger will work. It's free, but would require lots of backups and probably a great deal of your time.
Use a commercial partition utility to merge the partitions without erasing the main partition.
iPartition, VolumeWorks, Drive Genius. Cost: $45 - $99, plus a small amount of time.
Get a new HD from newegg. Your 5 year old G4 will have a drive no bigger than 20 GB. It will be slow, noisy, and possibly getting close to failure compared to modern drives. $61-$62 for a new 120 GB drive including shipping. Modest amount of your time to format, do clean install, import users, move anything across that the user importer missed. This is probably the best option since you need to do it anyway.
Leopard may come with an updated Drive Utility or Boot Camp that can undo partitions. No guarantees. We don't know for sure that Leopard will support the older G4s either. $100 - $150.