What exactly is London/Decca

I'm hoping someone may enlighten me on what exactly a London/Decca "Phase 4" record is. At first, I thought it must be a quadraphonic record, but I've picked up a few and they aren't. Most seem to be from the late 60's. One came with a promotional insert that describes it as a "marvel of sound" (not particularly enlightening). Might anyone know what the difference is between a "Phase 4" London/Decca and any other non-Phase 4 London/Decca record? Tks.

John