I was a SAHM for the first 6-9 months after my sons were born and, since I'm a teacher, I have summers home with them. Here's what I learned, for what it's worth...
Yahoo has a great calendar function to replace Outlook if you don't have that at home. Does your cell phone have a calendar app? Now would be a great time to find out. But, overall, my family uses a paper/pencil weekly calendar that lives on our island. House rule: if it's not on the calendar, the family is not committed to the event.
When my sons were babies, I had a very tight schedule and it worked very well. Now that they're older (13 and 17), we still have a schedule, although it's more flexible. I would encourage you to check out www.flylady.net. This might be a great start for you to keep some sanity and order to your life. (I do this during summer when I'm off school.)
Since I live in rural Nevada, and gas is increasingly expensive, errands are one day a week. My boys and I usually make a day of it (we call it "going to town"), complete with lunch out, household errands and, if they've been good, a "fun" stop, usually Borders or Wal Mart. We also usually do one "field trip" day a week. This could be going to a museum, the beach, a baseball game, something out of the ordinary. This gets us out of the house two days a week and we all work hard at spacing them out so we don't get sick of each other.
I guess the biggest thing I've learned about my time at home is that it requires far more discipline to stay organized and on top of things at home than it ever does at school. My hats are off to full-time SAH mamas! The second biggest thing I learned is how valuable outside play time is to wearing my kids out so they sleep well at night. It doesn't have to be structured; simple "go outside and play" works well. The third biggest thing I learned during my time at home is not to be afraid of boredom (for the kids). My kids know that if they say, "Mom, I'm bored," I'll find them chores to do. Over the years, my sons have discovered all sorts of cool activities on their own with this philosophy. At 4, your daughters might be young for chores, but I never backed down from saying "find something in your toy box to play with." They have to learn to entertain themselves.
I hope this helps. Good luck to you!