Spectra 2 has the cooling fan in the backbox, spectra 3 has the cooling fan in the dome, Specra 4 doesn't have a cooling fan. So spectra 4 dome can be place in a Spectra 2 back box with reduce capability. Spectra 3 dome can go into a Spectra 2 backbox. Spectra 2 dome can not be used in a Spectra 3 or 4 backbox because there is no cooling in those back boxes