4-6 months is actually a great age. We went with both of ours when they were 5 months, so I have some experience here. No, they won't really notice anything (though our oldest did perk up a lot at the Nemo ride into the Seas Pavilion), but they're also super easy at this age. Don't need solids yet so only have to worry about breastfeeding or bottles; I have experience with each as only my youngest was still BFing at that age--so easy to do at the parks and no clean up. And at that age, if you are BFing, you're both well established into a good pattern so it's not as difficult as it was earlier on. Get yourself a good carrier like the Ergo, and he or she will be perfectly content at that age just to travel on you and nap on you while you're on the go. As far as sleeping, USUALLY they're sleeping through the night at that age or down to one feeding a night if you don't mind some mild sleep coaching. Truly, it's so easy at that age versus waiting until they're older babies or toddlers. Otherwise, I think about 2.5 yrs starts becoming a good age again. 6 months to 2.5 yrs was a bit rough for us in terms of travel.