If Trump does stand in 4 years time, lets say as an Independent candidate, the Republican party is going to have a real problem.
If they do not choose him as a their candidate he will stand anyway as an independent as take millions of votes away from them and potentially lose them the 2024 election.
They then may be forced to choose him as their candidate and so risk either splitting the party who may want to move on to somebody else or risk losing the election, as Trump may be viewed by the American public as an old hat – has been.