You don't need to be 4.5' deep in the yard or untraveled areas. I'm in northern Indiana, and it will never freeze that deep unless the ground is severly compacted. However, if you go under a driveway or anywhere the soil is compacted, 4.5' may not be deep enough.
I've dug up yards in the dead of winter, and under a few inches of snow, the dirt won't be frozen more then about 12" deep, but I have seen it freeze 4' under driveways in extended cold snaps. Personally, I would go about 4' deep, and you will be fine. And if you decide to go that extra 6", trust me, that is one heck of a lot of digging for a lousy 6", especially over 650' of distance. The first 3-1/2 to 4' of digging is the easy part, and once you get beyond that, it is a lot of dirt moving and much, much more work. Plus it's a lot harder to climb in and out of the trench!
As for pipe, personally I wouldn't use PVC pipe. I've seen too many problems over time, especially if the pressure exceeds ~ 60 PSI. It doesn't like the continuous stress of these pressures, and eventually something fails. Now if you want to use Sch 80, you will be fine, but that is $$$$. I would only use either Polyethelene with the highest available pressure rating, typically 160 PSI, or better, PEX pipe. PEX is the best, but could get pricey for that distance, and isn't really required for cold water.