I have a calendar that states my holidays and vacations each year. I give it to my families on the first of the year. I take the same two weeks off over the year every year. The week of July 4th and a week over Christmas.
I do take an occassional day off here and there through out the year like you mentioned but I only give 1-2 weeks notice depending on what the reason is I am taking off. For example, over Easter, I noticed I only had 2 kids scheduled for the Monday after so I closed and told the 2 families scheduled the week prior.
I can't help when things come up and I try my best not to put too many people out at once but it happens. Stuff comes up and I try to make it a rule that I will always do one week minimum at the very least so I think your plan for 4 weeks is more than generous!
ALL my families are required to have back up care too. I think that as long as you are flexible and don't close a ton, most families are ok with a day here and there. Last year I took 3 unexpected personal days with as little as 1 week notice and as much as a month so I don't think that was excessive.