  We report on the serendipitous discovery of a z=4.0, M1500=-22.20
star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (~60%
escaping), a remarkable multiple peaked Lya emission, and significant Lya
radiation directly emerging at the resonance frequency. This is the highest
redshift confirmed LyC emitter in which the ionising and Lya radiation possibly
share a common ionised cavity (with N_HI<10^17.2 cm^-2). Ion3 is spatially
resolved, it shows clear stellar winds signatures like the P-Cygni NV1240
profile, and has blue ultraviolet continuum (\beta = -2.5 +/- 0.25, F_\lambda~
\lambda^\beta) with weak low-ionisation interstellar metal lines. Deep
VLT/HAWKI Ks and Spitzer/IRAC 3.6um and 4.5um imaging show a clear photometric
signature of the Halpha line with equivalent width of 1000A rest-frame emerging
over a flat continuum (Ks-4.5um ~ 0). From the SED fitting we derive a stellar
mass of 1.5x10^9 Msun, SFR of 140 Msun/yr and age of ~10 Myr, with a low dust
extinction, E(B-V)< 0.1, placing the source in the starburst region of the
SFR-M^* plane. Ion3 shows similar properties of another LyC emitter previously
discovered (z=3.21, Ion2, Vanzella et al. 2016). Ion3 (and Ion2) represents
ideal high-redshift reference cases to guide the search for reionising sources
at z>6.5 with JWST.
