My personal suggestion would be to play "school!" And maybe get a schedule in place. That would be a great way to get them ready for school. I would have the same schedule for all your kiddos but then have individual "mommy time" with each or pair them up, where you can do something that is age-appropriate for each child. Take a look at this to see some tips about toddler schedule ideas. Here are some more ideas for your four year old here.
I have a 4 yr old, a 2 yr old and an 8 month old. They watch a LOT of TV. I know it is not good for them, but I don't know how to keep them entertained. I don't want to ban TV altogether, but I need ideas on what else they can do. The problem lies in their ages. My 4 yr old can only do crafts in my bedroom with the door closed. If my 2 yr old is left alone with a crayon even for a second, he will color all over the house. Only the baby naps. Also, my 4 yr old can't have anything on the 1st floor of the house that is a choking hazard for the 8 month old who is crawling. I don't want them playing rough (which they do often) when I'm busy feeding the baby, so I turn on the TV. They also watch TV while I cook. So, basically, I'm having trouble juggling everything so I rely on the TV. They are in Pre-K twice a week, which helps. They also enjoy play doh and pop-beads at the kitchen table. (I still have to supervise the 2 yr old with small objects.) I don't take them places by myself much because they are very difficult to manage by myself.
For my hubby some easy ideas that he likes are play dough, which you already do and playing with food! That's something that we do for fun. Use carrots, broccoli and crackers to build houses and other critters. This is easy and fun because you don't have to worry about them eating crayons or the play dough. Something else would be to involve them in your chores. While the little ones play somewhere near, you can have your older one dry the plastic or non-breakable dishes. Or while your 4 year old watches her favorite show you can all sit down and match socks and have the little ones practice putting them neatly away. Again a schedule would be nice and have them put check marks or stickers when they've successfully helped you do a chore. Seek out kid friendly parks, with fences where you can go as a treat. Play grocery shopping and teach them how they should act when they go out shopping with mommy! Join them while they play for a few minutes a day. Be a kid again and you to will have just as much fun.
I hope these tips will help you and others out when you run out of ideas and keep them away from the front of the television as well. Oh and speaking of television. There are some great websites, like Starfall.com that can keep your kids entertained and learning as well. They can also learn from the shows they watch, but point things out as well. Make sure they are interacting, not just staring at the "idiot box." Play games, like have them point out all the red things on TV. Designate time frames and what shows they can watch and for how long. Usually we watch one 2 hr movie as a family and maybe 30-45 mines a few times a week.
Best of luck!