Hi! I'm not a HS momma, but I am a public school teacher by trade (I SAH right now). While I'm not going to HS (officially, though I do a lot of guided activities at home right now b/c we all enjoy it) I can tell you in honesty that since you're doing 1:1 or 1:2 that your guided/direct teaching time can be a lot less and be just as effective. Four hours a day for preK is overkill, IMO. Not that it's BAD, but really not necessary. If you're both enjoying it and there is no frustration in either of you, that's fine, but again, not really necessary.
If I were HS and had to put a daily time frame on guided lessons, it would look like this (broken into 2 "sessions" per day as they get past KDG):
preK - maybe 15 - 30 minutes a day
K - 30 min - 1 hour
1st - 1 - 2 hours
2nd/3rd - 2 - 3 hours
4th/5th/6th - 3 - 4 hours
7th - HS - 4 - 5 hours
Then, add in whatever social extras you do as a HS family (zoo, museums, music lessons, dance, sports, etc...) and unguided time (independent reading, handwriting practice, educational computer time, free play, etc...) and you will have a well-rounded day in MUCH less time than the public school day b/c your classroom will be more efficient (I've always said that I'd LOVE to teach a class of no more than 10 kindergartners rather than the 20 I have had in the past!
).
Anyway, I hope I'm not butting in. I have toyed with the idea of HS, however at this point I will have to return to work FT in less than two years for the income, so HS isn't in the cards right now. We definitely will continue our supplemental activities though. GL with your journey!