I'm a retired Electrical Engineer and had done a lot with early audio, the CFD244 is giving you good advice, use a 8 ohm speaker, the radio's from Bendix will not tolerate 4 ohms the transistors of that time are not that robust, if you really want 4 ohms speaker there are mods to make it work, but that would require changing the output amplifier stage or a 2:1 output impedance matching transformer. A modern I/C Amp it would increase easily increase output power and wouldn't care what speaker impedance is, they're designed to handle it.