Scaling topological charge in the CP$^{3}$ spin model

The CP^3 spin model is simulated at large correlation lengths in two dimensions. An overrelaxation algorithm is employed which yields reduced critical slowing down with dynamical exponents z around unity. We compare our results with recent multigrid data on the massgap m and the spin susceptibility and confirm the absence of asymptotic scaling. As a new result we find scaling for the universal topological susceptibility with values extrapolating to chi_t / m^2 = 0.156(2) in the continuum limit.