Update from ESA Mission Director Thomas Beck at ATV-CC:
At 09:20 CEST today, ATV-4 used two of its four main engines (M1 and M3) to lift the ISS into a slightly higher orbit. After a burn of 185 secs, the velocity of the orbital complex has been increased by 0.48 m/sec. The amount of propellant burned for this manoeuvre was 68.68 kg.
The next reboost using ATV is planned for 13 September 2013.