Here is what is on there site. I also sent them a message to see how long until they have the 4600's in stock. What do you think about the 4300's?:

The factory we buy our packs from is having issues with IR(internal resistance) on some models. This is creating some backorders and delays before we can get more packs in stock as we must wait until new bathes of cells are ready. Basically packs with higher IR will have lower performance. This may not be noticeable to everyone but we take pride in selling packs with a certain level of performance and we feel it's best not to sell these models until the issue is resolved. The 4670-2S2P shorty pack is one of our most popular models and we have many customers who have these on order or asking when more will be in stock. We had to sell the last shipment 4600s that came in as 4300-50C packs as the IR was higher.

We received a new shipment of 4300-50C from a different factory and the mAh is very good. We're seeing 4600+ on a iCharger when discharged at 10 amps. The IR of this new 4300-50C is a bit lower than the previous batches of 4300-50C but not as low as the good batches of 4600-70C packs. I believe this will make for a very good pack for the price as we're offering it for 29.95

For updates on backorders or info in general you should check out our Facebook page as we do post info as it becomes available.