Well remembered but fairly difficult to find although it sold well enough to be findable, the Pm-4 was part of the second run of ESOTEC components, 1980, therefore still (partly) designed in the USA and sporting the old style serial numbers and seeing only very very briefly non-Esotec tagged versions.
What everybody remembers is the shape of its enclosure : a sort of mix between the Yamaha M-4 and the 1979 Marantz Ma-5 (or 1968 Model 18). This look was attempted at earlier by Hitachi with the HMA-3790 and HMA-4590.
As can be seen from the image below the pre-production sample was planned cheaper : instead of one-piece diecast side heatsinks, the fins were much thinner and screwed one by one to the main aluminium sides...
Inside is a fair preamp section (nothing stellar, though) and a Class A output section with 90MHz hi-fT transistors, switchable in Class AB.
Low-TIM, low-NFB design, low-noise/high-gain phono MC inputs, short signal paths and OFC wiring.
Other than that... Marantz didn't advertize for it much... but everybody remembers the Pm-4 nevertheless.
A real Pm-4 at hifi-do's.