The Constitution can be amended. Not that I would support that happening, in this case... I feel like the Democrats would amend the Constitution, to allow Ah-nold Schwarzenegger to become President, and help him to run as a Democrat, against Trump - because they would rather have a rightwing candidate who could perhaps theoretically win against Trump, and that thry could control - than an actual leftist, with a better chance of winning, as with Sanders in 2016 (before he tarnished his own name and legacy, proving that he was controlled opposition repeatedly).
FDR was elected to 4 terms.