What model PowerBook G4, and which screws? At the cost of labor ($20-40 an hour depending on the shop) you can almost as easily buy another PowerBook and just swap your goodies in. Unless you've got a DLSD or 1GHZ Titanium, it's likely you can get a good working PowerBook G4 around $100, often less.

If it's just lower case screws and you're feeling ballsy? Use a Dremel with a diamond cutting bit (like a ball head 3mm) and take the heads off the screws, remove the lower case and use stripped-screw puller pliers to grip the shaft and remove headless screws. You can replace the damaged lower case for $10-30 off Ebay for almost any PowerBook G4.

Pliers: https://www.amazon.com/Powerbuilt-Multi-Function-Screw-Remover-Pliers/dp/B0763VJL52/