  Charge symmetry breaking (CSB) in the $\Lambda$-nucleon strong interaction
generates a charge dependence of $\Lambda$ separation energies in mirror
hypernuclei, which in the case of the $A=4$ mirror hypernuclei $0^+$ ground
states is sizable, $\Delta B^{J=0}_{\Lambda}\equiv B^{J=0}_{\Lambda}
(_{\Lambda}^4{\rm He}) -B^{J=0}_{\Lambda}(_{\Lambda}^4{\rm H})=230\pm 90$ keV,
and of opposite sign to that induced by the Coulomb repulsion in light
hypernuclei. Recent ab-initio calculations of the ($_{\Lambda}^4{\rm
H},{_{\Lambda}^4{\rm He}}$) mirror hypernuclei $0^+_{\rm g.s.}$ and $1^+_{\rm
exc}$ levels have demonstrated that a $\Lambda - \Sigma^0$ mixing CSB model due
to Dalitz and von Hippel (1964) is capable of reproducing this large value of
$\Delta B^{J=0}_{\Lambda}$. These calculations are discussed here with emphasis
placed on the leading-order chiral EFT hyperon-nucleon strong-interaction
Bonn-Juelich potential model used and the no-core shell-model calculational
scheme applied. The role of one-pion exchange in producing sizable CSB level
splittings in the $A=4$ mirror hypernuclei is discussed.
