quartus

Quartus, quarta, quartum is the ordinal to quattuor, and declines like bonus. Notice that the double t of quattuor does not survive into the derivatives: quartus, quadraginta, quadrans, quadratus all show a different treatment of the same root, so do not try to spell quartus with two t's on the strength of the cardinal.

Latin tells the time with an ordinal and hora in the ablative, no preposition: quarta hora, "at the fourth hour". That is roughly mid-morning, but only roughly, because the Romans split daylight into twelve hours whatever the season, so a winter hour was short and a summer one long. English quarter, quart and quartan fever all descend from it.