We report on the serendipitous discovery of a z = 4.0, M-1500 = -22.20 star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (similar to 60 per cent escaping), a remarkable multiple peaked Ly alpha emission, and significant Ly alpha radiation directly emerging at the resonance frequency. This is the highest redshift confirmed LyC emitter in which the ionizing and Ly alpha radiation possibly share a common ionized channel (with N-HI < 10(17.2) cm(-2)). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni NV lambda 1240 profile, and has blue ultraviolet continuum (beta = -2.5 +/- 0.25, F-lambda similar to lambda(beta)) with weak low-ionization interstellar metal lines. Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6 and 4.5 mu m imaging show a clear photometric signature of the H alpha line with equivalent width of 1000 angstrom rest-frame emerging over a flat continuum (Ks-4.5 mu m similar or equal to 0). From the SED fitting, we derive a stellar mass of 1.5 x 10(9) M-circle dot, SFR of 140 M-circle dot yr(-1) and age of similar to 10 Myr, with a low dust extinction, E(B-V) less than or similar to 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 reionizing sources at z > 6.5 with JWST.
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