hi i have a dji f40 with naza v2 and i m looking to buy some more battery for it should i go for 3s or 4s

and i want that have longer flight time

thank you

What motor prop combo are you using?
Make sure they are 4s compatible.
With the stock 2212 DJI motors we usually run 3 s on a lightly loaded copter and 4S if we have more accessories such as landing gear and cam/gimbal.
Flight time will be influenced by to Amp hours, Prop type and size and total weight including batteries.  Higher voltage does not give you longer flight time per se but gives you  the ability to run a higher prop speed and lift heavier loads

thank you your information

this is the kits i have

DJI Innovations Flame Wheel F450 Kit ARTF.
Quadcopter Frame kit which includes Built in distribution board
4x Motors 2212 920kv E300 3s - 4s
4x 15A E300 OPTO Esc's
4 x Propellers New Self Tightening

1x Naza V2 flight controller with GPS module

and i just order the iOSD mini + 2.4G Bluetooth Datalink + CAN-Hub and will put this in as well, also i have a gimabl and cam as well

so do you think i better to have a 4s battery is better

I wold agree you will probably be better off with 4S.