  The same-sign tetralepton signature via mixing of neutral Higgs bosons and
their cascade decays to charged Higgs bosons is a unique signal in the type-II
seesaw model. In this paper, we study this signature at future lepton
colliders, such as ILC, CLIC, and MuC. Constrained by direct search,
$H^{\pm\pm}\to W^\pm W^\pm$ is the only viable decay mode for $M_{A^0}=400$ GeV
at $\sqrt{s}=1$ TeV ILC. With an integrated luminosity of $\mathcal{L}=8~
\mathrm{ab}^{-1}$, the promising region with about 150 signal events
corresponds to a narrow band in the range of $10^{-4}~\text{GeV}\lesssim
v_\Delta \lesssim10^{-2}$ GeV. For heavier triplet scalars $M_{A^0}\gtrsim 900$
GeV, although the $H^{\pm\pm}\to \ell^\pm \ell^\pm$ decay mode is allowed, the
cascade decays are suppressed. A maximum event number $\sim 16$ can be obtained
around $v_\Delta\sim4\times10^{-4}$ GeV and $\lambda_4\sim0.26$ for
$M_{A^0}=1000$ GeV with $\mathcal{L}=5~ \mathrm{ab}^{-1}$ at $\sqrt{s}=3$ TeV
CLIC. Meanwhile, we find that this signature is not promising for
$M_{A^0}=1500$ GeV at $\sqrt{s}=6$ TeV MuC.
