  The energy spacing between the ground-state spin doublet of $^4_\Lambda
$He(1$^+$,0$^+$) was determined to be $1406 \pm 2 \pm 2$ keV, by measuring
$\gamma$ rays for the $1^+ \to 0^+$ transition with a high efficiency germanium
detector array in coincidence with the $^4$He$(K^-,\pi^-)$ $^4_\Lambda $He
reaction at J-PARC. In comparison to the corresponding energy spacing in the
mirror hypernucleus $^4_\Lambda $H, the present result clearly indicates the
existence of charge symmetry breaking (CSB) in $\Lambda N$ interaction. It is
also found that the CSB effect is large in the $0^+$ ground state but is by one
order of magnitude smaller in the $1^+$ excited state, demonstrating that the
$\Lambda N$ CSB interaction has spin dependence.
