This is the right forum! This place just isn't terribly busy and it can take a bit to get responses depending on who is on and the nature of the question.

It's difficult to say what could have happened to the frogs. Four of them vanishing is pretty alarming, and the first things which come to mind are predation or disease. Even though you might not have herons, it's possible something else came by and got them, though I'm not sure what animals may be in your area. Disease may be unlikely since you would have seen bodies if they got sick and died. I would try to do a sweep of your pond and check the little nooks and crannies to see if they did pass and get wedged in them, if to only try to figure out their fate. Keep an eye on the remaining two for signs of illness. Just in case it is a predator, it may be a good idea to get a net to drape over the pond, even if it's just at night.