As someone else posted - based on your "so what happened" update, it's not clear that you're really open to what folks are suggesting here; you did ask for opinions but don't seem to want to reconsider that half hour of "desk work" each day.
Sure, she's been doing nicely with it -- so far. But now the novelty is wearing off for her. She has been enjoying it -- to a degree. But did you consider that much of that enjoyment has likely come from knowing she was pleasing you, not from any innate love of "desk work"? Please consider that these factors are now what you're dealing with -- the novelty is gone for her, and she was pleasing you but now that's worn off too.
Four year olds who do pencil-and-paper worksheets daily may seem to be getting ahead, but you may be setting her up for resisting you and even resisting her kindergarten teacher next year. You clarify that you are doing lots of creative play, outdoor play, etc., but she's four. At that age they are starting to assert themselves and their wants. All the play time in the day won't make her more accepting of the desk work if she has begun to resist it. And frankly if you turn it into a discipline situation --do the desk work or you'll get this consequence -- you risk really turning her off this type of work, which she will need to do willingly in just a few years, and you don't want her associating it with discipline.
At four, the activities you mention she already does are her learning opportunities: Helping in the kitchen becomes math as she counts the grapes, adds or subtracts them, measures things, etc. Playing outside is science: She collects things, you find out their names, she writes simple labels for her own "museum." If she's a good writer, let her make her own grocery list for the next time you go to the store together (spellihg does not matter). Reading with her, which you're already doing, will help her learn to read better and better. Have her listen to books on CD at home and in the car -- that builds her imagination and comprehension. Take her to every museum you possibly can, even ones that aren't "kids' museums," and do a little research in advance to help her enjoy them as you tell her about things on her level. Folks who posted earlier had plenty of other great ideas.
I also would consider a preschool. I don't know if you have a specific reason you're not sending her to one, but preschool is about much more than the academics. Children in kindergarten are expected to be able to be absolutely ready to do the following:
Listen to, and obey, an adult who is not a parent or caregiver they know well.
Switch from activity to activity quickly and without fussing or resisting (it's time to leave art and go to story time, or it's time to stop playing and put away the toys, etc.).
Get along with other children and learn not only to share, but also to cooperate.
These and other social and group skills are ones that a good preschool helps a child develop, because when the child gets to K, the teacher has no time to work on this. Schoolteachers now are so pressed to fit in specific curriculum during specific "instructional time" that they can't do a lot to help a child learn these basics. So even if your daughter is good as gold, shares on playdates, and likes other adults, being in the school-like setting of a preschool can help her learn these skills before kindergarten, in ways that I think can't be taught at home because she's not part of a peer group at home, and she's not being told to do things by an adult who is not a parent.
If you don't like the replies you're seeing here, try asking an elementary teacher if daily desk work, however short, is age-appropriate at four, even for a very bright four-year-old. See what he or she says and go with that, whatever it is.