Most tree-style hydrangeas are paniculatas as they really get quite tall. Let the soil remain moist well m lched and try not to overdo the watering. No need to fertilize until next year if the potting mix came those round fertilizer pellets.

Just so you cover all bases and just so your warranty remains valid, call them now and register "the problem" in case you end up having to return it. But frankly, I would not consider it "a problem" yet as they probably grew it in weird conditions and it is simply slow leafing out.

A paniculata planted on the ground should have started leaf out in Zone 5 by now (but I am not sure of Zone 4 though). Flower buds will be visible much later, in mid to late June (there are some paniculatas that will have flower buds real late in July).

Luis