I recall reading an article some time ago by one of the project developers at Phillips at that time. While the specs for quad allowed flexibility on how it would be utilised, the thinking was that it would be the same bit rate but with 40 minutes content, ie the same as a quad LP. As to why it was never utilised, I'm guessing that there wasn't a big enough market for quad recordings to justify the effort and developing a capable CD player. Also, around the time it was removed from IEC specs, SACDs and DVD audio had greater capability than four channel, eg 5.1.The only quad CD's I'm aware of are the DTS encoded ones. I don't believe there's enough room on a CD for uncompressed quad
DTS, as far as I'm aware, never met redbook specs and those releases were not able to display the compact disc logo.