The interplay between the crystalline and magnetic structures of a 4% Cr-doped Bi0.37Ca0.63Mn0.96Cr0.04O2.99 has been investigated by alternating current (ac) magnetic susceptibility, electrical resistivity, and neutron diffraction measurements. The compound crystallizes into a monoclinic P2(1)/m symmetry. A Jahn Teller distortion occurs at 280 K. The thermal behavior of charge transport may be described by a three-dimensional variable range hopping conduction. Strong interplay between the localized magnetic electrons and the itinerant electrons are clearly revealed as the localization length increases by 20% when the Mn spins become ordered below 85 K. Short range magnetic correlations persist up to 160 K. The collinear magnetic structure can be viewed as consisting of ferromagnetic spintrimers antiferromagnetically embedded in a ferromagnetic environment. Cr-doping reduces the charge ordering temperature and the magnetic ordering temperature. It nevertheless introduces long-range ferromagnetism.