I remember back at the times of 256/512/1gb rules the usb drive capacity market 2-3 years ago, my fren also got a 4gb sony from China. we were also "Whoa!" that time.

2 months down the road when my fren copied more and more data into it (approx. 2gb++) the drive start to give problem. I still remember clearly that it is 77 days the drive keep it alive, after that it RIP.

the drive is slightly more expensive than the (that time) newest 1gb...

Conclusion. I doubt the 400gb capacity.