As far as internal upgrades, what's your budget?
There are a ton of different things you could do internally and I would definitely recommend doing them.
IIRC, the G4 series are built around G&G Combat Machines. In the CM, the electrical components and gears are rather weak compared to other guns in the price range. I would look at a Lonex Red (A1) motor first. This is their balanced motor, with a great balance between torque and speed that will do a lot of good for your ROF and trigger response. Next I would look into a rewire. After that, look into a gear set change. For a higher ROF/trigger response you'll want a lower ratio gear set. The standard ratio is 16:1, so you'll want to stick with those (get Lonex gears though), or look into 13:1 "high speed" gears. SHS and Core make good 13:1 gears. Modify also makes them but they are subpar in quality. Finally, your piston. The piston also plays a large role in the ROF and trigger response. Look into the SHS 15T or the Lonex Red. I personally use a Lonex Red and love it, you can get them prelightened (holes drilled in them, making for a lighter piston allowing for a higher ROF) for a rather cheap price over on Hivemind airsoft.