The fourth complement of a given interval is its interval distance from the fourth (4/3). It's very similar to the fifth complement in that it can be thought of as a way of conceptualizing and constructing triads, albeit such triads are naturally not of the same class as those framed by the more familiar perfect fifth for a number of reasons. Furthermore, the idea of a fourth complement seems to be very useful in regards to tetrachords, and even in helping to describe certain relationships in functional harmony such as "parallels" in which notes in the same tetrachord located a third apart from each other share some aspects of their harmonic function.
Aside from the usage of the term "fourth complement" on this wiki, the term was brought up by Aura in a discussion on Discord concerning reasons that the relationships between Perfect and Augmented Fourths is not entirely analogous to the relationship between major and minor seconds in MOSes despite being separated by the same interval.
The following interval pairs are fourth complementary to each other