I'm probably in the minority but most rules from home get thrown out the window at WDW. I of course still expect dd5 and dd8 to use their manners and be polite, no whining, etc. but rules such as bedtimes, breakfast foods, etc. don't exist at WDW. We just go with the flow, stay up late for the fireworks, get up early for rd, napping in the stroller if needed, we eat ice cream for breakfast some days, etc. To me it just makes vacation that much more special when the girls get to do stuff that they don't at home.
Now that won't work for all kids or families so I really think that you have to do what is best for you and your kids. I probably wouldn't do that with an 18 mo. old like OP has, but maybe I would if the baby would sleep in the stroller... IDK.
If baby would sleep in the stroller, OP is probably not stuck in the hotel room. You could take your 4 yr old swimming while baby slept (of course 1 parent would need to sit with baby poolside), OP and her dh could take turns taking dd4 to MK after dinner while the other stayed in the room with the baby, etc. There are all kinds of possibilities.
I think what ends up happening is what works best for each family, something that works for one is a failure for another. Great to get ideas but bottom line, you'll only figure out what works best for you once you are actually at wdw and are doing it
I hope you have a wonderful trip!