De är kumulativa: de lägger till varandra, ersätter inte varandra.
I avsnittet Pathfinder SRD på Multiclassing :
Instead of gaining the abilities granted by the next level in your character's current class, he can instead gain the 1st-level abilities of a new class, adding all of those abilities to his existing ones. This is known as “multiclassing.”
For example, let's say a 5th-level fighter decides to dabble in the arcane arts, and adds one level of wizard when he advances to 6th level. Such a character would have the powers and abilities of both a 5th-level fighter and a 1st-level wizard, but would still be considered a 6th-level character. […] He adds all of the hit points, base attack bonuses, and saving throw bonuses from a 1st-level wizard on top of those gained from being a 5th-level fighter.
(Betona min.)
Så som en 4-nivåskämpe som nivellerar upp och tar en nivå av Rogue, skulle du vara en Fighter 4 / Rogue 1, en 5: e nivå karaktär totalt och dina sparar skulle vara + 4 / + 3 / + 1 .