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  A slightly modified & explainified Shulgin Scale:

Technically it follows three so four is not wrong, in that you are unlikely to encounter a +++* (+4) below the full +3 effects of a (psychedelic) drug,

+++* (+4) is a rare condition where the effects of the compound serve only as one (pre-requisite) piece of what unlocks access to such states. 

The heterogeneous fourth symbol being a glyptoglossic gimmik that reminds us that The Shulgin Scale is 

not a four category system; +1 through +4. 

With a bonus sixth category +++* or (+4), a rare, significant, special state of affairs which has been occasionally encountered, where the effects of the drug seem to….uuuuuum yeah. 

It is wise advice that once one has acquired at least two of something (be it white powders, +++* experiences, or anything really) it’s prudent to create a label.