Persistent nuclear burning in Nova Sgr 2016 N.4 (=V5856 Sgr = ASASSN-16ma) six years past its outburst

We report on the fast Nova Sgr 2016 N.4 being surprisingly trapped in a long-lasting and bright plateau (Delta I >= 10 mag above quiescence) six years past the nova eruption. Very few other novae experience a similar occurrence. We carried out an intensive observing campaign collecting daily BVRI photometry and monthly high-resolution optical spectroscopy, and observed the nova in ultraviolet and X-rays with Swift at five distinct epochs. The bolometric luminosity radiated during the plateau is similar to 4200 L-circle dot (scaled to the distance of the Galactic Bulge), corresponding to stable nuclear burning on a 0.6 M-circle dot white dwarf. A stable wind is blown off at full width at zero intensity (FWZI) similar to 1600 km s(-1), with episodic reinforcement of a faster FWZI similar to 3400 km s(-1) mass loss, probably oriented along the polar directions. The collision of these winds could power the emission detected in X-rays. The burning shell has an outer radius of similar to 25 R-circle dot at which the effective temperature is similar to 7600 K, values similar to those of a F0 II/Ib bright giant. The Delta m < 1 mag variability displayed during the plateau is best described as chaotic, with the irregular appearance of quasi-periodic oscillations with a periodicity of 15-17 days. A limited amount of dust (approximate to 3 x 10(-11) M-circle dot) continuously condenses at T-dust similar to 1200 K in the outflowing wind, radiating L-dust similar to 52 L-circle dot.