The operating system on your G4 must be as old as the machine, that it still includes Classic. The new OS comes out in November, which would replace both Classic and your OS with just the new OS. However, just how much hard disc space you will recover...I doubt it will help if you're so short you're having to delete individual programs. So even finding a way to delete Classic may well not solve your problem. If you don't want the full expense of the new OS, then with the new OS coming out, Amazon sellers will probably be selling OS 10.4 dirt cheap, so you get to update your OS somewhat and get rid of Classic when you install it.
The second possibility costs even more money: buy an external storage disc. How big you need will depend on what you are saving - video eats up 4 to 5 GBs a pop. And I can't remember whether the G4 has Firewire - essential for quick reading and saving.