Re: How come most .308's I see are 4+1 capacity...?
I guess it boils down to what rifles you are looking at and whether or not you are willing to install some sort of DBM setup for use with larger capacity (more than 4-5 rounds) magazines.
You won't find many factory production bolt action rifles with anywhere near a 10-round capacity. In fact, the only one that comes to mind off hand would be the Ruger/Gunsite Scout rifles which come with 10-round mags.
Production being defined as your average Remington, Savage, Ruger, etc. hunting rifle bolt action...not to include rifles like the Sako TRGs, AIs, etc. which have a bit more of a specialized goal/purpose in mind.