Can I use my old mac mini (G4) as an optical drive with new mac mini late 2014?

I think so

DVD or CD sharing: Using Remote Disc - Apple Support

You should also be able to connect the two via FireWire using a FW 400 cable plugged into the old Mini, thne a FR 400 femial to FW 880 adopter plugging into a FW to Thunderbolt adopter

How to use and troubleshoot FireWire target disk mode - Apple Support

Just confirmed this can be done between G4 running 10.4.11 and mac mini running 10.10. No problems talking between each other.