We present a new measurement of the Λ4H hypernuclear lifetime using in-flight K− + 4He → Λ4H + π0 reaction at the J-PARC hadron facility. We demonstrate, for the first time, the effective selection of the hypernuclear bound state using only the γ-ray energy decayed from π0. This opens the possibility for a systematic study of isospin partner hypernuclei through comparison with data from (K−, π−) reaction. As the first application of this method, our result for the Λ4H lifetime, τ(HΛ4)=206±8(stat.)±12(syst.)ps, is one of the most precise measurements to date. We are also preparing to measure the lifetime of the hypertriton (Λ3H) using the same setup in the near future.