I'm thinking that 4 speakers rated at 4 ohms, all wired parallel on a mono radio will require a 1 ohm stable external amplifier.

If the factory 70 charger rear speaker fader would split the mono radio into front and rear channels, then you could get a two channel amp and put 2 speakers on each channel. 2 speakers rated at 4 ohms would let you run a 2 ohm stable external amplifier. Should be cheaper and easier to find.

Tav