Hi Matthew,
The magic that you see through your children's eyes at Walt Disney World will be amazing. We took our daughter for the first time when she was 7 months old and many times since, so I can speak from experience on taking little ones to Walt Disney World!
I think that one of the keys to a happy vacation is keeping everyone on their own schedule as much as possible. For your 4-month-old, if she needs a nap, either you or your wife could go find a quiet place for her to rest while the other adult visits attractions with your son. Often a shady bench or even an air conditioned show works well as a place to rest. Each theme park also has a Baby Care Center. These centers include quiet places to feed your baby, changing stations, and items such as diapers and wipes for purchase if you forget them. For your 4-year-old, a trip to Walt Disney World, while completely amazing, can also be overwhelming. Just like your infant, keeping him well-rested and happy is key. You may want to build in some down time in the afternoon. This may mean taking in some shows, sitting down for a snack, or even heading back to your resort. We are big fans of arriving at a theme park first thing in the morning (as most young children are early birds!), having a great morning at the theme park, and then heading back to our resort in the early afternoon for some relaxing. Finally, consider getting a stroller for your son. Even though 4-year-olds are fairly capable of walking through their everyday lives, there is a lot of walking in a theme park. Being able to save him some of those steps will save him some energy!
I hope that this information helps, Matthew. Have a wonderful trip, and please do not hesitate to ask if you have any further questions!