  We present an explanation of the 4th branch of the Z-track based on analysis
of high-quality RXTE data on the source GX 5-1. Spectral analysis shows that
the physical evolution on the 4th track is a continuation of the flaring branch
which we previously proposed consists of unstable nuclear burning of the
accretion flow on the neutron star. In flaring there is a huge increase of the
neutron star emission from a volume that increases to a radius of 21 km. The
4th branch is shown to consist of flaring under conditions that the mass
accretion rate and thus the total source luminosity is falling. We detect
strong emission on the flaring and 4th branches at energies between 7.8 - 9.4
keV inconsistent with origin as Fe K emission, which we suggest is the
radiative recombination continua (RRC) of iron Fe XXVI at 9.28 keV and of lower
states. Evolution of the emission takes place, the energy falling but the flux
increasing strongly, consistent with production in the large volume of unstable
nuclear burning around the neutron star which eventually cools.
