Primarily internet slang, and used as an acronym for "The fourth plateau," the highest level of intoxication, hallucination, and dissociation from the body when using the drug dextromethorphan, or DXM.
"Man, last night was crazy! I reached t4p for the first time on an 1100mg dose!"
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).