  We revisit models where a heavy chiral 4th generation doublet of fermions is
embedded in a class of two Higgs doublets models (2HDM) with a discrete $Z_2$
symmetry, which couples the "heavy" scalar doublet only to the 4th generation
fermions and the "light" one to the Standard Model (SM) fermions - the
so-called 4G2HDM introduced by us several years ago. We study the constraints
imposed on the 4G2HDM from direct searches of heavy fermions, from precision
electroweak data (PEWD) and from the measured production and decay signals of
the 125 GeV scalar, which in the 4G2HDM corresponds to the lightest CP-even
scalar h. We then show that the recently reported excess in the $\gamma\gamma$
spectrum around 750 GeV can be accommodated by the heavy CP-even scalar of the
4G2HDM, H, resulting in a unique choice of parameter space: negligible mixing
(sin\alpha ~ O(0.001)) between the two CP-even scalars h,H and heavy 4th
generation quark and lepton masses m_t',m_b' < 400 GeV and $m_{\nu'},m_{\tau'}$
> 900 GeV, respectively. Whether or not the 750 GeV \gamma \gamma resonance is
confirmed, interesting phenomenology emerges in q' - Higgs systems (q'=t',b'),
that can be searched for at the LHC. For example, the heavy scalar states of
the model, S=H,A,H^+, may have BR(S -> q'q') ~ O(1), giving rise to observable
q'q' signals on resonance, followed by the flavor changing q' decays t'->uh
(u=u,c) and/or b'->dh (d=d,s,b). This leads to distinct high jet-multiplicity
signatures, with or without charged leptons, of the form q'q' -> (nj + mb +
lW)_S (j and b being light and b-quark jets, respectively), with n+m+l =6-8 and
unique kinematic features. It is also shown that the 4G2HDM can easily
accommodate the interesting recent indications of a percent-level branching
ratio in the lepton-flavor-violating (LFV) decay $h \to \tau \mu$ of the 125
GeV Higgs, if confirmed.
