  We complete the analytical determination, at the 4th post-Newtonian
approximation, of the main radial potential describing the gravitational
interaction of two bodies within the effective one-body formalism. The (non
logarithmic) coefficient a_5 (nu) measuring this 4th post-Newtonian interaction
potential is found to be linear in the symmetric mass ratio nu. Its
nu-independent part a_5 (0) is obtained by an analytical gravitational
self-force calculation that unambiguously resolves the formal infrared
divergencies which currently impede its direct post-Newtonian calculation. Its
nu-linear part a_5 (nu) - a_5 (0) is deduced from recent results of Jaranowski
and Sch\"afer, and is found to be significantly negative.
