In my opinion outside of them both being 60 range TDs are incomparable as they serve 2 different sub roles.
SU-85 is meant as all-rounder TD with heavy armor counter focus with low mobility, but high reliability, especially against heavies and is a top tier TD for soviets so that reliability is required as there is nothing above it.
JP4 is meant to poke primarily medium armor and TDs(given its self spotting without crippling mobility and own target size), it does not need to engage heavy armor, because that's specifically what panther is meant for.
Great theory but it has flaw that make it completely wrong.
Su-85 can counter PzIV (meduim) and the Panther (TDs) at least as cost efficient (if not more) as the JP4 can deal with mediums or TDs.
The units can be compared, and the SU-85 is simply not "much worse" than JP.
In addition SU-85 has higher mobility than the JP4 and the difference in target size is small (1).
Finally the SU-85 does not "have" to engage the mediums , because that's specifically what SU-76 is meant for.
Given stats of both, they clearly aren't meant to engage primarily the same types of targets.
And both are good for their designated roles-SU combating heavy armor and JP combating all medium.
That is you theory-crafting about single purpose roles. In game both units, once built, they might have to deal with mediums or heavily armored tanks. In either case the SU-85 is simply not "much worse" than the JP4.
Cost can not be used as an argument as OKW pays 25 fuel extra for everything medium and above as a tribute for other perks of faction(while not combat focused, vet5 still provides much more utility, especially for vehicles compared to other factions).
Once more that is you theory-crafting.
(unless you find a a reference from Relic that:" OKW pays 25 fuel extra for everything medium and above as a tribute for other perks of faction" it did not happen.)
Cost can and should be used when 2 units are compared.