I'm not so sure this is the good deal we want it to be...... If you look at the rate details it doesn't actually show that you will get a fourth night free. The discount seems to be incorporated in the daily rate itself. Your warning about the vagueness of the rate description might be even more relevant.

When I look at the dates I'm traveling I get a rate of JPY 36k per night. But It seems you have to pay for all 4 nights. When not using the special rate code I also get a rate that states 4th night free, but that rate is JPY 59k and specifically mentiones the 4th night will not be charged at checkout.

So it seems this special rate still is a better deal but the price per night will be around JPY 37k including taxes. And it's a prepaid rate...on the bright side you will get credited a 4 night stay instead of a 3 night one.