We report on the detection of two series of harmonically related doublets in IRC +10216. From the observed frequencies, the rotational constant of the first series is B = 1380.888 MHz and that of the second series is B = 1381.512 MHz. The two series correspond to two species with a ²Σ electronic ground state. After considering all possible candidates, and based on quantum chemical calculations, the first series is assigned to MgC₃N and the second to MgC₄H. For the latter species, optical spectroscopy measurements support its identification. Unlike diatomic metal-containing molecules, the line profiles of the two new molecules indicate that they are formed in the outer layers of the envelope, as occurs for MgNC and other polyatomic metal-cyanides. We also confirm the detection of MgCCH that was previously reported from the observation of two doublets. The relative abundance of MgC₃N with respect to MgNC is close to one while that of MgC₄H relative to MgCCH is about ten. The synthesis of these magnesium cyanides and acetylides in IRC +10216 can be explained in terms of a two-step process initiated by the radiative association of Mg⁺ with large cyanopolyynes and polyynes followed by the dissociative recombination of the ionic complexes.