Checkout the ProCraft Backpack. Super comfortable and roomy for P4, accessories and more! Only $119
Yes, it's nice, I just got a ProCraft myself ($120). If you carry a tripod all the time, this pack would be almost perfect, because it's got the handy fold out tripod pocket. However I personally think they don't need the 6 internal Velcro straps. I'll likely end up cutting them off because it's just too much hassle keeping them cinched and organized. My ThinkTank Helipak ($200) backpack for my other Phantom doesn't have the straps, and that pack has worked perfect, it's a no hassle design. There are also straps on the outside of the ProCraft to securely hold the tripod, but they also get in the way to open and close the primary lid zipper. I may cut those off too since I don't carry a tripod.
I kinda like the Helipak design better, maybe because we've been so many places together...

but it's certainly not worth the extra $80 over the ProCraft IMHO. The ProCraft looks like a clone to the Helipak layout, but it actually has better lid compartments storage layout, and also some handy removable zippered packs (which I'll likely never use, it's just another zipper to open) along with a good waist belt for long hikes. There are a few things I like about the Helipak better over the ProCraft. 1. It's slightly thinner (1/2") and lighter, plus it's got a second grab handle on the side that I use all the time. The Procraft has only one grab handle on the top. The Helipak has a better primary lid zipper, it's more robust and easier to zip.
Between these two, either one can get the job done with tons of room inside. Both can be configured to individually protect 3 or 4 batteries in separate compartments. Other compartments hold all of the other accessories like chargers, props, filters, iPad
Air2, sun shade, MarcoPolo tracker, wind meter, windsurfers, etc. Plus they can both hold a DJI RC with the smaller Itelite aftermarket antenna installed, which is nice. I'm not sure if any other pack can do that, but maybe. Both have nicely padded shoulder straps and waist belts. I have a 50X optical zoom point and shoot (Canon SX50) that I often throw in, because it has the extra room. The ProCraft has two outside pockets for water bottles, Helipak has only one, because the second grab handle is sewn in that area.