Quartan

a.	1.	Of or pertaining to the fourth; occurring every fourth day, reckoning inclusively; as, a quartan ague, or fever.
n.	1.	(Med.) An intermittent fever which returns every fourth day, reckoning inclusively, that is, one in which the interval between paroxysms is two days.
	2.	A measure, the fourth part of some other measure.

Adj.

1.

quartan - occurring every fourth day (especially the fever and weakness of malaria); "quartan malaria"

period, period of time, time period