If you will be in Disney for 10 days, I suggest that you plan on more
than 4 days in the parks. I generally tell people that they can
experience most of the rides and attractions in Animal Kingdom and
Hollywood Studios in one day each, but Magic Kingdom and Epcot require
two days each.
I definitely understand that there is a limit to how much anyone budget can stretch! But if you look at
Disney ticket prices,
you can see that going from, say, 4 days in the parks to 6 days will
only add $16 to the cost of your ticket. And because your two youngest
children will be free, that means you will only be paying for part of your family. If it were me, I would spend the extra $36 on tickets and my family would eat a cheaper meal to make up the price difference!
My own daughters are very close in age as well, and we've visited Disney with them since they were newborns, so I do understand the challenges of traveling with little ones. But as long as you have a stroller where they can rest or nap, I think your children will do very well, even for a few more days. With all that magic around them, why wouldn't they? :)