Billy,
re. the DX-4D
I think, yes. It was just an easier way to market the dbx products to the 6 different models that they were designed for at the time.
Namely the 22-4, 34, 38, 44, 48, & 58
Many musicians/recording studio owners at that time were trying to piece their studios together as they could afford it and TASCAM knew this. They were on top of their game back then in trying to offer a variety of different machines to fit to different needs and budgets.
The previous generation of 8 track machines called the 80-8 did have a singular 8 channel dbx unit but, I don't think they sold too many of those as a quick look at the e-bay sales of those machines would show that the majority of the recorders for sale did not have the dbx units to go with them. When you look at the 38's for sale used, there is a higher percentage of them with the dbx units as part of the package so, it can be un-scientifically deduced that TASCAM's marketing department knew what they were doing back then.
When I bought mine, used, back in the early 90's it came with both dbx units and even the remote control as well as the very rare rack ears! All for the normal used street price of $1500.00 CDN. The machine with all the accessories when new, sold for $4600.00 with all the extras.
Cheers!