iPhone 4 boot loop, no DFU possible
I could fix it with some luck now.
I didn't restart by holding power+home for 7 seconds.  

I pressed them for about 5 seconds, until the screen turned off.
Then I held the home button, and connected it to a pc withouth iTunes installed. (This didn't work with a pc with iTunes!)
Now it asked me to connect to iTunes, so I connected it to another pc having iTunes installed.
After iTunes finished restoring, the phone worked perfect again.