In order to build polaronic ferromagnetic polymers, poly(m-phenylenequaterthienylene) (PmPhTh₄) was designed and successfully prepared by electrochemical polymerization of 1,3-bis(2,2′-bithiophen-5-yl)benzene. The electronic properties of neutral and doped PmPhTh₄ were investigated by both experimental and theoretical methods. It was confirmed that the polaronic spins generated by the doping were confined to the neighbourhood of the quaterthienylene unit. For comparison, the electronic properties of the previously reported PmPhTh₂ were also examined.