Just to make this completely clear – "Phase 4 Stereo" had
absolutely nothing whatsoever to do with quad. It was a purely
stereo technique invented in
1961 by Decca (in fact there was even a mono variant called Sound 4 monophonic). It was designed to increase the impact of the new-fangled stereo system by closely miking (and over-emphasising) certain instruments.
A former Decca engineer explains here:
Decca - Phase 4 Stereo
Because the "Phase 4 Stereo" recordings made extensive use of multi-track recording (in order to spot light those specific instruments) there was plenty of scope for remixing them latter. It would appear that the Decca affiliate London records did that some years later, re-releasing a few titles in ‘quad’ format (confusingly) under the "Phase 4 Stereo" banner on tape.
To add to the confusion a couple of producers used the term "Phase 4" in connection with some matrix quad releases in quad in the early days of the format before the terminology and branding settled down a bit - but such use had nothing to to with the trademarked "Phase 4 Stereo" system of Decca.