The highest version of the Mac OS that runs on PowerPC Macs is 10.5.8. The last iteration of OS 10.4 was 10.4.11.
As 10.5 has more overhead than 10.5, I doubt it will make any speed differences and may knock speed down a notch or two. It did on a G4 tower I have with specs similar to the last of the G4 iMacs.
Things that help:
1) Max out the RAM. If you post more details about your iMac from Apple menu > About this Mac > More Info... we can see how much more RAM you model can accomodate (different G4 iMacs have different RAM specs; with modern modules, some can double the max amount of RAM the outdated Appel Specs call out).
2) See if the hard drive is getting too full. Although it varies with hard drive sides, for legacy Macs you want no less thant about 15 percent of the hard drive space free.
3) In OS 10.4, run the periodic maintenance scripts manually. Link :Running the Mac OS X maintenance scripts
If it's web video that's slow, you can't fix that. Most web developers use modern video technology that has abandoned any efforts to be optimized for pre-Intel mac processors.