B.,
I agree that your kids need time with you, but you also need time to focus on something other than them. I saw another mom mentioned FlyLady. You should really check her website out. Her site is free, membership is free, and you adapt the routines to fit your lifestyle.
One day a week FlyLady does a weekly home blessing hour. She literally cleans her home from top to bottom in one hour. This is something your 4-year-old can help with. The cleaning hour is just the basics (remember it doesn't have to be perfect!), then throughout the week you focus on your zones (1 area of the house at a time) and work for only 15 minutes in your zone at a time.
I can't make the weekly blessing hour work for me on just one day, so instead I break up the list and do one thing each day. This helps to keep the house looking "mostly" presentable and my sanity under control. Really, I can get my home ready for company in about 30 minutes now, where before it would take days, and I couldn't always find everything that I "put away" in bags and stored in the office. Now, I have a place for everything and can easily make the decisions on the mail as it comes in daily. This helps to keep the clutter down to a minimum.
The best part about the FlyLady system, you will be teaching your children how to clean the house and keep it fun! Especially if you include them in things they can do (like dusting the tables). You will show them how to be organized with a control journal (your routines written down in check list form), and how housework doesn't have to take all the fun out of their mom.
Your children will benefit from having a daily routine, knowing you do certain things at certain times of the day. By scheduling in house work, playing with the kids, and "me" time you will be a better mother and teach your children how to keep their lives balanced as they grow up.
And that's one of the most important parenting lessons. The one that will keep them sane when they become adults and start their own lives.
Good luck!