S.S.
Most of the other ladies had some advice on how this is a phase of life, and it is. I've been a SAMH for 13 years! My best advice, tell her how much fun she's going to have, but don't tell her you'll miss her a lot....tell her you'll see her when preschool is over in the afternoon (or whenever) and that you're going to have fun, too. Then find something that will occupy your time while she's away. Start a project that you can do while your 2 year old is napping, that you wouldn't normally be able to do, because you have a 4 year old underfoot (like painting a room, or sewing something, or meal planning, etc.). Make sure it's something you'll enjoy, and treasure that "me" time. :)
That's my best advice. I'm sure you'll do just fine, and so will she. We moms have been doing this for ages! LOL! It will also making her starting kindergarten next year that much easier for both of you.
Pick a hobby, and have fun doing it. Or maybe plan a nice lunch with a girlfriend once a week. Believe me, the time will fly.
Good luck!
S.