The dynamics of the gravitational two-body problem at fourth   post-Newtonian order and at quadratic order in the Newton constant
  We derive the conservative part of the Lagrangian and the energy of a gravitationally bound two-body system at fourth post-Newtonian order, up to terms quadratic in the Newton constant. We also show that such terms are compatible with Lorentz invariance and we write an ansatz for the center-of-mass position. The remaining terms carrying higher powers of the Newton constant are currently under investigation. 
Stefano Foffa and Riccardo Sturani
gr-qc