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A recent study of the positronium atom showed that the energy levels could be determined numerically using an effective field theory just as easily as the underlying true theory. Here, we expand on this idea by modeling the three-body positronium-ion and four-body di-positronium molecule with low-energy effective field theories. We then compare the results obtained using the effective theories to those found in a similar manner with the true theory to determine if numerical calculations will converge more quickly using the effective field theory due to removal of the Coulomb divergence. Finally, with the necessary framework in place, we calculate a matrix element needed to evaluate the correction to the magnetic moment of the positronium-ion due to the interaction between the positron and the two electrons which form a spin singlet.