Eles são cumulativos: eles somam e não substituem um ao outro.
Na seção SRD do Pathfinder em Multiclasse :
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.
(ênfase minha)
Então, como um lutador de 4º nível subindo de nível e pegando um nível de Rogue, você seria um Fighter 4 / Rogue 1, um personagem de 5º nível, e seus testes seriam + 4 / + 3 / + 1 .