In supersymmetric (SUSY) models with radiatively-driven naturalness (RNS),
the heavier Higgs bosons $H,\ A$ may have TeV-scale masses with the
SUSY conserving $\mu$ parameter in the few hundred GeV range.
Thus, in natural SUSY models there should occur large heavy Higgs boson
branching fractions to electroweakinos, with Higgs boson decays to
higgsino plus gaugino dominating when they are kinematically
accessible. These SUSY decays can open up new avenues for discovery.
We investigate the prospects of discovering heavy neutral Higgs bosons
$H$ and $A$ decaying into light plus heavy chargino pairs which
can yield a four isolated lepton plus missing transverse energy
signature at the LHC and at a future 100~TeV $pp$ collider.
We find that
discovery of heavy Higgs decay to electroweakinos via its $4\ell$
decay mode is very difficult at HL-LHC. For FCC-hh or SPPC, we study
the $H,\ A \to $ SUSY reaction along with dominant physics
backgrounds from the Standard Model and devise suitable selection
requirements to extract a clean signal for FCC-hh or SPPC with
$\sqrt{s}=100$ TeV, assuming an integrated luminosity of 15 $ab^{-1}$.
We find that while a conventional cut-and-count analysis yields a
signal statistical significance greater than $5\sigma$ for
$m_{A,H}\sim 1.1-1.65$ TeV,
a boosted-decision-tree analysis allows for heavy Higgs signal discovery
at FCC-hh or SPPC for $m_{A,H}\sim 1-2$ TeV.