If I have a masterwork climbers kit would I get a +4 bonus to my climb checks?
Well there's no price for a masterwork climbers kit in the core book, but if you sweet talked your GM into being able to buy/craft one it likely would add +4.
At least that's what the precedent for all the other masterwork items provides.
There's some actions where you take a -2 penalty if you don't have the correct items for it. Such as trying to craft with no tools or open a lock with no thieves tools. Climbing though you can do without anything at all. So the climbers kit gives you a +2 normally rather then negating a penalty.
The masterwork thieves tools technically give you a +4 bonus since it starts at a -2.