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The new evolutionary code Pegase.3 is used for the spectral synthesis ofcontinuous optical-Spitzer-Herschel-submm spectral energy distributions (SEDs)of two z=3.8 radio galaxies 4C 41.17 and TN J2007-1316. Both galaxies areselected from the Herschel Radio Galaxy Evolution Project for their faint AGNcontribution and because they show evidence of a large stellar contribution totheir bolometric luminosity. Pegase.3 coherently models the reprocessing of thestellar luminosity to dust emission, allowing to build UV to IR-submm librariesby types. These templates allow to predict SEDs at redshifts z in theobserver's frame by using z=0 templates corrected for cosmology and evolution.Our best fits are a sum of two distinct evolving stellar populations: i) a verymassive starburst observed ~30 Myrs after formation from optical and far-IRpeaks ii) a significantly older (~1Gyr old) massive early-type populationrevealed from mid-IR Spitzer data. The AGN emission fits a roughly thermalmodel. These results confirm that many of the stellar populations in high-zradio galaxies were already formed by massive (a few thousands of billion solarmasses) starbursts at z=4, confirming the previous K-z diagram interpretation.Gas-rich mergers and/or jet-cloud interactions are favored for triggering theintense star formation, possibly in relation with massive super black holes ofthese radio galaxies. These multiple stellar populations may be a genericfeature of the luminous infrared radio galaxy population as a whole andstrongly constrain mass accumulation and galaxy formation models.

Starburst and old stellar populations in two z=3.8 radio galaxies 4C 41.17 and TN J2007-1316