I'm a residential contractor, and use the 100ft. - 500 ft. rolls of black 4" and 6" often. I have never had a problem with it cracking or leaking, but I use it for drainage, so I don't really have a way of checking it for leaks. It is very tough though. As for PVC, it is only as good as it's glue joints. If you are using PVC in 10 or 20 ft. sections, you will need a really straight ditch. With the price of Schedule 40 in 4" or 6", a person would be better off paying the price for the Thermopex.
I used 1" oxygen barrier pex, with closed cell foam sleeves slid on from the end, not slit. (I had to spray the pipe with silicone spray to get them on) I then wrapped it in 3 wraps of HVAC foil backed bubble wrap. I slid it inside a 6" black corrigated pipe. I can't imagine trying to get it inside a 4" pipe, especially if you have to go through a 45* or a 90*. If I do get a leak, neither of these forms of insulation will absorb water, so there will be little chance of heat loss due to wet insulation. As an added measure, I filled the ditch with sand around and to the top of the pipe for further protection, and cut 2" thick high density foam similiar to "Tuff-R" and laid on the sand before the dirt went in. I feel very confident about my pipe installation.
I may have more money in mine than if I had purchased the prepared pipe, but I see posted often that heat loss equals firewood. I plan on installing the pipe only once,,,,,,,,,,, firewood is every year.