Well Plumbing question
I don't believe there are many regulations about buried water pipe. Best practice in most areas is at least 18&quot; from the top of the pipe, to the top of the ditch.   You  are definitely future proofing your install, but getting a second pipe of same size will be a very snug fit and probably logistically challenging. It's also very unlikely for a pipe to &quot;fail&quot; unless mechanically damaged.  Once in a while a coupling will fail and start leaking if it wasn't glued up well.   But those can be found (think wet spots!), dug up and repaired. Same with mechanical damage.  Surely protecting the 1.5&quot; pipe inside a 4&quot; would vastly minimize the chances of  mechanical damage.   But if you are so concerned about it, why not just run 2 1.5&quot; pipes now? 4&quot; pipe would be quite expensive and hard to install the 2nd line at a later date.   BTW, what type of pipe? PVC? Poly?   One more thing: If PVC, you could run schedule 80, which is pretty tough stuff.