It is normal for hydrangeas to wilt/droop in the Summer, specially if the soil is not moist enough. Their leaves loose moisture faster in the Summer than the roots can absorb. And so they wilt... especially a newly planted hydrangeas... You can give them extra water in the Summer although be careful with that as too much water can result in root rot if the roots are in wet soil for too long of a period. Not sure how old the tree is but it sounds as if it cannot absorb enough water quickly enough or is not getting enough water. Make sure your waterings get the soil moist down to a depth of 8". The others obviously have a better/larger root system and can handle the temps. Things that can help on the short term are probably not always too practical with the trees: increasing the water to Summer levels; an umbrella/chair/etc to give them some extra shade; extra water, which you are already providing; blocking the winds if it is in a windy area, etc. It is a normal problem in soils like Florida (which are sandy and therefore drain too well); 5-10cm of mulch all year long past the drip line (I assume it already has this); etc. So just let it wilt. It should recover on its own by the morning. I have some non-trees that do this in the Summer. There is little I can do as temperatures hover at 107F/42C so I just make sure the soil is evenly moist and let them wilt. If it becomes quite troublesome, perhaps consider transplanting in the Fall to get sllightly more shade but, do remember that they should handle hot temps better as they get older.

When was this tree planted?

The fact that other hydrangeas are doing fine 'in the same conditions' suggests either this tree is newly planted or something is wrong with it. An established paniculata in Zone 4 should not be wilting that much.

Yes, I totally agree with pennlake's comments, an established paniculata is pretty tough and should not be wilting unless it's really rather dry. I'm also in zone 4 and these plants have been troupers!