  The intrinsically hole-doped RbEuFe$_4$As$_4$ exhibits bulk superconductivity
at $T_{\mathrm{sc}}=36.5$ K and ferromagnetic ordering in the Eu sublattice at
$T_\mathrm{m}=15$ K. Here we present a hole-compensation study by introducing
extra itinerant electrons via a Ni substitution in the ferromagnetic
superconductor RbEuFe$_4$As$_4$ with $T_{\mathrm{sc}}>T_{\mathrm{m}}$. With the
Ni doping, $T_{\mathrm{sc}}$ decreases rapidly, and the Eu-spin ferromagnetism
and its $T_{\mathrm{m}}$ remain unchanged. Consequently, the system
RbEu(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ transforms into a superconducting ferromagnet
with $T_{\mathrm{m}}>T_{\mathrm{sc}}$ for $0.07\leq x\leq0.08$. The occurrence
of superconducting ferromagnets is attributed to the decoupling between
Eu$^{2+}$ spins and superconducting Cooper pairs. The superconducting and
magnetic phase diagram is established, which additionally includes a recovered
yet suppressed spin-density-wave state.
