Bicycle Mechanics - 4mm vs 5mm diameter derailleur cable housing

I went to order some derailleur cable housing and noticed that it is available in both 4mm and 5mm diameters. Are there specific applications or shifting systems that use one versus the other, or how do you determine which is appropriate for your application?

Thanks

I always thought the 4mm is for derailleur housing and 5mm for brake housing. Where are you ordering from?

My kids' Comfort bikes and Mountain bikes are cabled with 5mm derailleur housing. My new "high performance" road bike has 4mm derailleur housing. I have recabled the kids' bikes and verified that it is 5mm coaxial derailleur housing. My bike shifts better with the 4mm housing.

It may be more a characteristic of the housing rather than the size. Indexed shifters need non-compressible housing. Common 5mm (brake) housing isn't non-compressible. The 5mm shift cable housing should be, but maybe it's not as good.

My kids' Comfort bikes and Mountain bikes are cabled with 5mm derailleur housing. My new "high performance" road bike has 4mm derailleur housing. I have recabled the kids' bikes and verified that it is 5mm coaxial derailleur housing. My bike shifts better with the 4mm housing.

Interesting. I've always felt that 5mm cable housing worked slicker but I still use 4mm on my go-fast bike because - well - it's a road bike and I'm a little obsessive compulsive..

All the "factory" shift cable housing that comes with Shimano STI and Campy Ergo brifter sets is 4 mm.

Thanks for the replies. I'm looking to order from AE Bike. The housing I'm looking at is the non-compressible type for indexed shifters. I'm ordering this to replace the housing on my Specialized FSR XC mtn bike.

Here are links to the 4mm and 5mm versions of the same derailleur housing;
http://aebike.com/page.cfm?PageID=30&action=details&sku=CA4300

http://aebike.com/page.cfm?PageID=30&action=details&sku=CA4301