does anyone has an idea how configure nginx as reverse proxy host like x4b.org does ? maybe via dns or ? for multiple sites.

As the owner of http://x4b.org/ I am probably the best person to answer this question.

We use a combination of nginx, many open source nginx modules (both modified and unmodified) and lots of proprietary glue in our HTTP stack.
Our TCP and UDP proxy stack is pure custom now days (expect exciting things!).
Our DNS system is a combination of open source and lots of custom glue.

I take from this that we must be doing something right if people are looking to copy us :)

Mathew