  Let $\mu$ be a self-similar measure generated by iterated function system of
four maps of equal contraction ratio $0<\rho<1$. We study when $\mu$ is a
spectral measure which means that it admits an exponential orthonormal basis
$\{e^{2\pi i \lambda x}\}_{\lambda\in\Lambda}$ in $L^2(\mu)$. By combining
previous results of many authors and a careful study of some new cases, we
completely classify all spectral self-similar measures with four maps.
Moreover, the case allows us to propose a modified {\L}aba-Wang conjecture
concerning when the self-similar measures are spectral in general cases.
