Context. The identification of the sources that reionized the Universe and their specific contribution to this process are key missing pieces of our knowledge of the early Universe. Faint star-forming galaxies may be the main contributors to the ionizing photon budget during the epoch of reionization, but their escaping photons cannot be detected directly due to inter-galactic medium opacity. Hence, it is essential to characterize the properties of faint galaxies with significant Lyman continuum (LyC) photon leakage up to z similar to 4 to define indirect indicators allowing analogs to be found at the highest redshift.Aims. Long gamma-ray bursts (LGRBs) typically explode in star-forming regions of faint, star-forming galaxies. Through LGRB afterglow spectroscopy it is possible to detect directly LyC photons. Our aim is to use LGRBs as tools to study LyC leakage from faint, star-forming galaxies at high redshift.Methods. Here we present the observations of LyC emission in the afterglow spectra of GRB 191004B at z=3.5055, together with those of the other two previously known LyC-leaking LGRB host galaxies (GRB 050908 at z=3.3467, and GRB 060607A at z=3.0749), to determine their LyC escape fraction and compare their properties.Results. From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of log(N-HI/cm(-2)) = 17.2 +/- 0.15, and negligible extinction (A(V)=0.03 +/- 0.02 mag). The only metal absorption lines detected are CIV and SiIV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Lyman-alpha (Ly alpha) emission. From its Ly alpha emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to 1

0.35(-0.11)(+0.10)

0 .

35

- 0.11

+ 0.10

(

0.43(-0.13)(+0.12)

0 .

43

- 0.13

+ 0.12

),

0.08(-0.04)(+0.05)

0 .

08

- 0.04

+ 0.05

(

0.08(-0.04)(+0.05)

0 .

08

- 0.04

+ 0.05

) and

0.20(-0.05)(+0.05)

0 .

20

- 0.05

+ 0.05

(

0.45(-0.15)(+0.15)

0 .

45

- 0.15

+ 0.15

), respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRBs presented here have all M-1600> -19.5 mag, with the GRB 060607A host at M-1600> -16 mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore, the time involved is minimal compared to galaxy studies.