PLUS FOUR (++++) A rare and precious transcendental state, which has been called a 'peak experience', a 'religious experience,' 'divine transformation,' a 'state of Samādhi' and many other names in other cultures. It is not connected to the +1, +2, and +3 of the measuring of a drug's intensity. It is a state of bliss, a participation mystique, a connectedness with both the interior and exterior universes, which has come about after the ingestion of a psychedelic drug, but which is not necessarily repeatable with a subsequent ingestion of that same drug. If a drug (or technique or process) were ever to be discovered which would consistently produce a plus four experience in all human beings, it is conceivable that it would signal the ultimate evolution, and perhaps the end, of the human experiment.

I’ve noticed that some members refer to any very intense psychedelic experience as a “+4”. This is not correct usage of the Shulgin scale. A +4 experience isn’t “just” a very intense experience. Here’s how Shulgin describes a +4:

I really like that last sentence.