You would have less body roll in an 84 4000. It shares some of its
suspension with the turbo-coupe (or maybe it's vice-versa?). 85 and
later cars are much softer in the steering and suspension. This
self-serving statement mentioned because I have an 84 for sale.
I have an 84 4KQ and had a few different 85 and 86 4KQs. While the gearing is lower and the car is lighter for the 84, and I much prefer it for track use over the later cars, I feel the suspension is pretty much the same across all the 4KQs. Perhpas you are feeling more the relative lightness and not suspension differences, or perhaps your 84 has some nice suspension goodies on it.....