@iamhanuman
Thanks for the reply......the run time which u have specified,is it for zippy or any 5000mah.Does the run time depend upon the discharge rate of the battery?
And even for longer run times,does the truck accommodate two batteries?
What setup do u suggest for a 20-30 min runtime?

20-30 minute runtime may be achieved by going to 6s and re-gearing for 35 mph or so.

Runtime has multiple variables. Tire choice, traction, weight, mechanical efficiency, driving style et cetera.

can i use this battery?
http://www.hobbyking.com/hobbyking/store/__8586__ZIPPY_Flightmax_5000mAh_6S1P_25C.html

I can get up to 20 mins of runtime on 'normal' driving with very few times triggering hard on fully charged Lipo. This is with normal fun driving. Not abusive bashing. This is possible because stock truck is geared somewhat low. As per iamahuman, if you abuse it, you will get 10+ mins.
Iamahuman is correct that runtime depends upon Tire choice, traction, weight, mechanical efficiency, driving style etc.
Its weight as I told you is less than competing trucks, those huge tire/wheels are one of the lightest as compared to comparable tire/wheels.
And the differentials are efficient because they are helical cut. etc.

Its battery tray is 138x51x49. So according to that, select the battery.

As i am a beginner i will be doing normal driving....and cani purchase this 6s battery from the sale?
http://www.rcindia.org/for-sale/clearance-sale/msg125207/#new

Thats too big as per the battery tray specs.
I will recommend 4s battery as you are a beginner. 6s will be too much on stock gearing.
On 6s, drivetrains wear faster on all rc trucks. mt4 g3, savage flux are bullet proof on 4s.
Kindly order 4s packs from hobbyking. Get hold of it and thereafter buy 6s for occasional abusive bashing.

okay!...its just that i have to wait 15-20 days for the battery to arrive....but i guess i dont have any other options now!

firefly got that right. The MT4G3 won't take very well to 6s bashing.

Some people(not firefly) don't realize that 6s can be more efficient than 4s when used correctly. If you gear for the same speed(same power draw), a system on 6s will draw less current than a system on 4s. That will mean lesser heat and more efficiency. That generally translates into pretty high runtimes. You can't slap in a 5000mAh 6s though as it'll be quite heavy and upset the balance of the car. People go for 3300mAh 6s packs or the like with low Kv motors (around 1400Kv).

The MT4G3 was made with 4s in mind. Stick to it.

Thanks for the advice
And does 20C batteries supply enough power,as i am planning to order these batteries
http://www.hobbyking.com/hobbyking/store/__15521__Turnigy_5000mAh_4S1P_14_8v_20C_hardcase_pack.html?

20 c is not enough and before buying do check if battery will fit the tray of the car. 

The battery fits perfectly according to the dimensions give by 1firefly2.
so should i take these?
http://www.hobbyking.com/hobbyking/store/__21004__Turnigy_4000mAh_2S_30C_hardcase_pack_ROAR_APPROVED_.html

Yes, higher voltages mean lower current draw for same power as power is the product of current and voltage. W=IxV. If you take a 6s pack, lower the gearing for similar speed, then current draw will be less along with lower I2R losses. Same thing can be achieved by use of lower KV motors as iamahuman described. Power & hence speed will be similar due to higher voltage.

See, the recommended continuous 'C' rating is ~40 for these kinds of trucks especially mt4 g3. But 20 C batteries for people have held up ok.

C ratings tell you how much current battery can discharge. For example for 20C 5000mah battery,
max continuous current = (20x5000)/1000 = 100A
if voltage of nominal 4s pack = 14.8V, then,
max continuous power = 14.8x100 = 1480W
For burst 30C, Power is = 14.8x150 = 2220W

These are around the rated motor powers stated by motor manufacturers (have to take ~90% efficiency into consideration for output)
So, theoretically they can lug these 1/8 scale monsters. But motor power draw can be from 0 to these calculated figures & not always these. For eg if you are climbing an incline at top speed, power draw will be more than if you are cruising on flat ground.
But the quality of batteries must be good.
You can get higher quality batteries from our rcindia friend spherehobbies. Not sure about their stock status.

I'd go for a minimum of 40C.

Thanks guys for helping me out!
Finally ordered the MT4G3:)
Will be receiving the package within two days(as said by rcskyhobby  people).Cant wait to put my hands on the truck:)