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The M4 Core Project with HST is designed to exploit the Hubble SpaceTelescope to investigate the central regions of M4, the Globular Clusterclosest to the Sun. In this paper we combine optical and near-infraredphotometry to study multiple stellar populations in M4. We detected twosequences of M-dwarfs containing ~38% (MS_I) and ~62% (MS_II) of MS stars belowthe main-sequence (MS) knee. We compare our observations with those of NGC2808,which is the only other GCs where multiple MSs of very low-mass stars have beenstudied to date. We calculate synthetic spectra for M-dwarfs, assuming thechemical composition mixture inferred from spectroscopic studies of stellarpopulations along the red giant branch, and different Helium abundances, and wecompare predicted and observed colors. Observations are consistent with twopopulations, one with primordial abundance and another with enhanced nitrogenand depleted oxygen.

The M4 Core Project with HST -- II. Multiple Stellar Populations at the Bottom of the Main Sequence