At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a Be9 target in order to study the Λ binding energy of light hypernuclei. Positively charged kaons were detected by a short-orbit spectrometer with a broad momentum acceptance at 0° forward angles with respect to the beam, efficiently tagging the production of strangeness in the target nucleus. Coincidentally, negatively charged decay pions were detected by two independent high-resolution spectrometers. About 103 pionic weak decays of hyperfragments and hyperons were observed. The pion momentum distribution shows a monochromatic peak at pπ≈133MeV/c, corresponding to the unique signature for the two-body decay of hyperhydrogen HΛ4→He4+π-, stopped inside the target. Its Λ binding energy was determined to be BΛ=2.12±0.01(stat)±0.09(syst)MeV with respect to the H3+Λ mass.