Unveiling the oldest and most massive galaxies at very high redshift

(Abridged) This work explores the existence of high redshift massive galaxies unveiled with Spitzer+IRAC, but missed by conventional selection techniques based on optical and near-infrared observations. To this end, we use the multi-wavelength imaging data available for the GOODS-South field, and select a flux-limited sample from the IRAC 3.6um image to m(AB)<23.26. We confine our study to the galaxies undetected by the optical HST+ACS imaging and close to the detection limit of the K-band image (K>23.5 AB). Our selection unveiled 20 galaxies on which we performed a detailed photometric analysis. For each galaxy, we built an SED based on optical-to-8um photometry to estimate the photo-z and to derive the main galaxies physical properties. The majority of the sample sources show degenerate/bimodal solutions for the photometric redshifts (Abridged). These can either be heavily dust-enshrouded (Av~2-4) starbursts at 2<z<3 with bolometric luminosities L(IR)>10^12 Lsun, or massive post-starburst galaxies in the redshift interval 4<z<9 with stellar masses of 10^11 Msun. One galaxy, the only source in our sample with both an X-ray and a 24um detection, might be an extremely massive object at z~8 detected during a post-starburst phase with concomitant QSO activity (although a lower-z solution is not excluded). Our investigation of Spitzer-selected galaxies with very red SEDs and completely undetected in the optical reveals a potential population of massive galaxies at z>4 which appear to include significant AGN emissions. These sources may be the oldest stellar systems at z~4. These, previously unrecognized, optically obscured objects might provide an important contribution to the massive-end (M>10^11 sun) of the high-z stellar mass function and they would almost double it (Abridged).