Not sure how much time 4 is but with a 4.8 volt 2000mah with 5 or 6 digital servos I get about 6...........................12 minute flights depending on what type of flying I'm doing. That's in a 68 inch 13 pound Extra. In a trainer with the same pack and 4 standard servos I can fly most the day. I do have a volt meter and check after every flight though. If your getting 4 then you may get 8??? I also have a field charger so I never let my packs get too low. If your happy with a 600mah then keep using it but when you need a new pack try something a bit bigger and see for yourself. Another thing to think about is what type of charger do you have and how much added time will it take to charge a bigger pack. My favorite charger with handle 6 batteries at a time but only charges to 1500mah. I now tend to use that as my trickle charger and use my other chargers for the main charge.