I have not had a chance to test the Yeah 4000 LiPo's to see if they meet their specifications, and unfortunately I probably won't considering their price and not being a sponsored tester.
I did buy a few of the GE Power 4000 25C and 5000 25C batteries (2 of each, in case I get a bad one) which seem to be exceptionally cheap. They cost me about $33 and $52 each (yep, you do pay a massive premium for those extra 25% capacity of the 5000 over the 4000) and the tests came up well.
Their capacities are 3800 mah and 4900 mah when tested both under a 5C and 25C load and they handled their max continuous load for their complete capacity without getting hot or doing anything else nasty.
Another nice thing with them is they use silicone wire for their balancer tabs and they secure those wires very solidly to the battery itself. I have had 2 balancer wires come off other batteries after extensive use of flexing the wire where they used some cheap stiff wires.
Considering I paid $90 for my ProTek 4000 30C, and testing it against the GE Power 4000 25C both discharged at 100A, the GE maintained 0.2v higher across the discharge and it costs 1/3 of the ProTek.
It would be interesting to see how the Yeah 4000's perform, but I doubt if they would be much better than the GE's, so unless their price drops dramatically I will never find out.