In SoA, the only weapons better than +3 are Carsomyr +5, Warblade +4, Cutthroat +4 & one of Drizzt's scimitars. Irenicus summons two baalors & two glabrezu in the end fight, but I don't remember if a +4 was required to hurt them (I think a +4 was only needed to hurt demogorgon & mariliths in ToB).
In Watcher's keep there's several +4 weapons, though... like staff of the ram & spectral blade that Jaheira can use.