I'm referring specifically to the fort ref and will save bonuses. say i'm a level 4 fighter, and thus have +4/+1/+1 respectively. i want to take a level in rogue for the +1d6 on sneak attacks, and looking at the rogue progression table, lv 1 is listed as 0/+2/0. so will i add 2 to my reflex saves or does it only increase TO 2? if i had a higher reflex save to begin with, would it revert upon multiclassing?
They're cumulative: they add together, not replace each other.
From the Pathfinder SRD section on 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.
So as a 4th-level Fighter levelling up and taking a level of Rogue, you'd be a Fighter 4/Rogue 1, a 5th-level character overall, and your saves would be +4/+3/+1.