The unexpected observation of eccess of events in the angular distribution of e−e+ pairs in various nuclear transitions has spurred a large interest, both experimentally and theoretically. This eccess has been interpreted as the possible existence of a new bosonic particle, the so-called X17, of mass around 17 MeV. In the present work, we investigate theoretically the possible effects of the presence of the X17 in the 3H(p, e+e−)4He and 3H(n, e+e−)4He reactions. For these processes it is possible to compute accurate ab initio bound- and continuum-states wave functions, so the existence of X17 can be unambigously revealed. Moreover, by exploiting the rich structure of the 4He spectrum, it is possible to determine its quantum number, as, for example, if it is either a scalar, a pseudoscalar, a vector, or an axial particle.