Iron Man 4 never materialized, and while no official reason has been provided, it was most likely a combination of various causes.
Tony Stark's role in team-up films exceeded the need for a fourth Iron Man solo feature in the Marvel Cinematic Universe.
Following Iron Man 3, Marvel focused on establishing new heroes, making way for famous figures such as Black Panther and Doctor Strange.
Iron Man was and remains one of the most recognizable characters in the MCU, which begs the question of why Iron Man 4 was never made.
Robert Downey Jr.'s Tony Stark launched the MCU and was the only hero to appear in two solo films before The Avengers (2012) concluded phase 1.
RDJ later headlined Iron Man 3 (2013) in phase 2, but a fourth film was never made
despite Tony remaining a prominent protagonist in the franchise for another six years until his death in Avengers: Endgame (2019)
Negotiations were apparently taking place around the release of Iron Man 3, so the notion appears to have been considered.