Thank you very much for your reply and help
Meanwhile I spend some more time with my collection of USB 3.0 sticks and guess what: same issue depending on the usb stick type.
Then I looked into the benchmarks I did 1-2 years ago with these sticks and my last PC (Ivy Bridge, Z77).
I found that some of the sticks indeed always performed super bad on 4k writes.
But most were reading slower (4k) but writing faster with proper speed (4k, 2-3mb/s). So the controller (hardware, driver) is part of the problem.
As slow 4k write definitively affects my work with external storage media I really have to look closer at new purchases 4k performance.
A real pity that manufacturers don't really look into such "details".
Luckily I found a hint what the exact reason could be: http://www.computerbase.de/forum/showthread.php?t=1540331&p=18222110#post18222110 (german, sry
The user refers to the wear leveling and mapping table being the reason for super slow writes even with new sticks.
Interesting enough this thread is about a new USB 3.0 stick Kingston HyperX Savage which shows exactly the same 4k write problem I am facing.
Thanks again for the feedback. I will live with the problem for now but will keep an eye on it and wont spend any money on products which ignore 4k write speeds