The PR4 Trail have a different sidewall and bead area construction to make them stiffer and better suited for longer suspension travel bikes like the Stroms and other adventure/tall rounders. The compounds are a slightly different ratio than the regulars, and slightly softer to give the same grip as the standard tyres when in a stiffer carcass. Michelin recommend a matching pair of either Trails or standard, but the front size is only available in trail in 19". There should be no safety concerns in mixing them, and no legal concerns, but best performance will come from a matching set.
Email 2wheelinfo@michelin.com to confirm, they respond quickly.