No, any tank will work.
It depends on what position you play entirely. The highest performing gun that still runs on CO2 is probably the Azodin Zenith, but this can only be an opinion.
unscrew the tank, this will vent any co2 that is in the marker out the asa
You cant, the CO2 that is in the marker is vented out when you take the tank off.
No, paintball co2 is much higher pressure.
No, you should only run the G4 on HPA.
It depends on the specific marker, pump has little to do with it.
A paintball gun needs CO2 (or HPA) to fire at all.
CO2, stored in the tank is highly pressurized. The paintball being shot is caused by it being pushed out by the rapidly expanding gas. CO2 also cycles and reloads the next paintball.
A Co2 gun can go up to 300 fps. Sling shoots only get a paintball up to 240.
you need Co2 to fire any paintball gun, it is what physically moves the ball out of the barrel.
Take it to a shop! if you want to do it yourself the normal way is to put the bottle on scales and fill it from a larger bottle to a fill weight. To be honest though CO2 is well out of date. A smart move would be to move onto compressed air.