Hi R.,
First, you have a lot going on. Stop beating yourself up.
Second, focus on being healthy... not dieting. If you're breastfeeding, then you shouldn't be dieting anyway.
Third, look for things that everyone can do together! Like... have a dance party with your kids! Turn on some music and everyone dances for 30 minutes. No sitting down! No pooping out! Everyone keeps moving for 30 minutes. Doesn't matter if you march, or swing your arms...whatever... just crank up the tunes and have some fun. It's a great way to spend time with your kids and set a good example of getting some exercise. Also, while the older ones are at school you can get out and walk with the younger two. Obviously on really cold days, not a great option. But as the weather starts to warm up the opportunities to get outside will present themselves more and more. One thing that really helps me is shutting off the TV! I have a policy the TV doesn't even go on until my daughter is in bed. Granted, she is only 17 months old. But, if you tend to have the TV on most of the day, think about carving out a couple of hours where the TV is off. You'll be more motivated to do other things.
You could also do some yoga. There are some great stretches that kids can do too! You can get your older kids involved. Stretching your muslces helps keep them lean and strong.
Fourth, start making healthy food choices. Go through your fridge and cupboards... how many REAL foods do you have? Really look at your food. Is it mostly packaged stuff? Or do you have foods close to their natural state. The closer your foods are to their natural state, the HEALTHIER they are for you.
www.nourishyourkids.com is a GREAT resource. It is run by a nutritionist AND a pediatrician! It mainly focuses on feeding your kids REAL healthy foods...but if you change the eating habits of your kids, then you will follow right along. Set a good example and keep everyone healthy.
Fifth, stay hydrated. Toss out the juice. Toss out the sodas, toss out the coffee. I know I know! But WATER is the BEST hydrator. It nourishes your body and helps flush out the stuff you don't need. 6-8 glasses a day. DRINK UP!
If you start with these steps, the weight will come off and you will be healthier all the way around. After 4 kids, the weight most likely will be a bit tougher to come off and your shape will most likely be a bit different. BUT, healthy is the primary goal. :-)
Good luck!