What do you think about this?

This is a mainline in the Petroff.  It is fully playable for both sides.

really? I thought the last move, c4, was rare...

http://www.google.com/products/catalog?hl=en&rlz=1C1CHKZ_enUS437US437&q=petroff+defense+book&um=1&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.,cf.osb&biw=1024&bih=673&ie=UTF-8&tbm=shop&cid=6102066637586595711&sa=X&ei=MvN1T9eHJKes2wWL07DHDQ&ved=0CFkQ8wIwAQ

I used to own this book when I essayed the Petroff.  This was before all of the Nc3!? lines became popular.  The material you list is the very first stuff he covered.  Highly recommend the book - only $13 on Google shopping.  Well worth the money :-)

Im not a big fan of the petroff myself, but I do know how to play the mainline. however, the move c4 is rare and I was curious about it

Nc3 is considered more aggressive than c4.  c4 is a highly positional style. 

This is an solid and easy defence has not much lines. But it often transposes to the 4 knight defese

And you often may seem this line too

In my opinion the russian defence is an interesting game to play but because of people not knowing the main line it goes to the booring lines above where nothing interesting happens

and lets not forget the trap in  the russian defence

CS, got a sec to look at the database.  Something I missed from earlier.  Your 7. c4 is one move too early.  If you play 7. c4, you allow ...Bb4+, which looks like a KILLER!  

7...Bb4+ = 18-30-52 with 33 games (365chess.com)

You need to play 7. 0-0 first, and then 8. c4. 

I know Bb4, but it doesnt look too bad. If played right, I think white gets out even