I agree-- 4mo is really just along for the ride. I'd think about transportation (if you can avoid the buses with strollers, that's easier... but if you can't, it's still manageable). And pool/playgrounds.
If you do end up using the buses (especially later at night), here's a tip:
As the infant gets sleepy, transfer him/her to a baby carrier (I use Baby Bjorn, but I hear Ergo Baby is great with infants). He/She will fall asleep there and mom and baby can walk onto the bus while you (or older child/other adult helper?) folds up the stroller.
I did this with my squirmy one-year-old on each bus ride and it helped, too, to keep him "strapped" to me during the ride (and not able to squirm off of my lap).
Also-- we stayed at a Kidani studio and put the PnP in the bathroom. MAKE SURE THERE IS NOTHING ON THE TOWEL RACK OR SHELF THAT BABY COULD REACH AND GET! We brought our video monitor so we could still see/hear him while we were on the balcony. We used the kitchen sink to brush our teeth. The only downside, of course, was the sprint to the lobby bathroom in the morning!