Try this, for now:
CurrentValue[$FrontEnd, TaggingRules] = GeneralUtilities`ToAssociations@CurrentValue[$FrontEnd, TaggingRules];
CurrentValue[$FrontEnd, {TaggingRules, "ChatNotebookSettings", "Model"}] = "gpt-4"
See Avery's comment for a more comprehensive answer. My code snippet will set "gpt-4" once and for all (at the $FrontEnd level, which is persistent across frontend notebook sessions)
