Yeah, I looked at the size within the shimano xtr kit and its 4mm so that's what I went with. Thanks for the reply. I wonder why they have the 5mm size if the cable is still the same size. Maybe it gives you some more room for the cable to move so that it doesn't get stuck on any buildup inside the housing.
I think the 5mm housing (O.D.) is generally used for touring bikes and bmx bikes. 5mm is the housing size of a brake cable housing. I'm pretty sure the 4mm was developed to address brake routing on frames with weird bends and angles. I'm sure it's a few grams lighter too.