I'm currently running 10.4.4 on my iMac, but I just recently bought Shake 4.1, and it says I need 10.4.6 or newer. So if I upgrade will I lose all my data like pictures, video, etc.?
10.4.4 to 10.4.8 (or 6) does not require re-installing your OS. If you run software update, it will automatically find, download, and upgrade your OS. The only way you would loose all your stuff is if something goes wrong when doing the upgrade. I think for some upgrades, Apple recommends backing your documents up.
you should always have a backup when making changes to your system. i've never had anything bad happen nor have i heard of anything bad happen to anyone i know personally (there are always a few stories you can find online).
if everything runs how it should run, you shouldn't lose anything.
Alright, well the thing is I use it for my editing computer so I don't have it plugged into the internet. So I can just go to Apple.com and look for the update? And download it to my PC, and transfer it to my iMac?
You can download it to your PC, make sure you download the correct version, there are a couple of different versions, PPC, Intel and Server. Going from 10.4.4 you will probably need to download the Combo update version.