Republican presidential candidate Odds
Political parties in the United States who hold the honor of possessing the Presidency have, for the most part with only a couple of exceptions, always run their leader at the top of the ticket in a reelection year.
While primary elections within a specific party still occur even if they control the Presidency, the results are pretty much guaranteed, and rarely feature influential or worthy challengers.
Based off of this fact alone, one can safely assume that in 2020, barring some sort of disaster, removal from office, or untimely demise, President Donald J. Trump will stand atop the Republican Party ticket as part of a bid to win a second term in office.
However, despite the odds being hugely in favor of Trump, there is still the potential (though extremely small) of a fellow Republican with enough of a grudge to launch an attempt to steal his party's nomination (perhaps current Speaker of the House Paul Ryan or another politician that is yet obscure and unknown).
This would be a long shot (and that is a serious understatement), but there is no specific clause in the Republican Party that prohibits such an action, and it would come down to registered Republican voters to determine whether the sitting President deserves the nomination once again.
And as an individual with a rather advanced understanding of politics and the trends that occur in this realm, I personally cannot see anything short of economic, military, or internal collapse leading to such a scenario.