My thought was the opposite, a four year old in crowds can keep up (but your point is good, just not what I thought of) - when the parks are empty you can take advantage of the ability to MOVE....fitting a lot more into a day simply because it takes less time to get from point A to point B - little legs might not keep up.
The whole stroller thing depends on the kid, the circumstances of the trip, the parents, the touring plan - all sorts of variables. Patient parents can coax a slow four year old strollerless though the parks. An active four year old might not need one at all. Parents with a packed touring plan, a kid who doesn't tend to move fast and tires easily, who are spending the vast majority of their time in the large parks (Epcot and AK) - will likely want a stroller.
I actually do agree with you - one can move more easily through a crowd without a stroller. Also, whether or not one needs one really
does depend on a lot of different factors. I probably should have clarified a little better, on past trips, we have gotten caught up in litterally a crush of people (ie, leaving the park) and it could have been a real nightmare to keep track of ds and make sure he didn't get trampled since people were shoulder to shoulder. Sometimes, those situations can be nearly unavoidable. In other situations, not having a stroller can definitely be an advantage.
The OP was also concerned about being in a monorail resort and having a stroller that folded easily. Tthis will only be an issue when
not taking the Monorail, as strollers can be rolled right onto the monorail.
We have taken our jogging stroller to WDW in the past, once while staying at SSR, so we had to take the bus everywhere since we didn't have a rental car for the majority of our stay. I simply made sure the stroller was already folded and ready to go before the bus got there.
Sometimes, the bus is waiting when you arrive at the bus stop. Unless it's full already, I've found the drivers to generally be pretty patient and will wait while you get your stroller folded so you can board (I know I would tend to get stressed thinking I was holding things up but I probably shouldn't have been so worried about it). Another thing we did was before leaving the park for the day, was to take all loose items (like those refillable resort mugs) that were thrown in the stroller basket into a canvas tote bag which we looped over the stroller handle. That way, when we left the park, we could simply have ds hop out of the stroller, grab the bag off the handle, fold, and go. Dealing with a stroller can be a hassle, but if you have a system down, it's not so bad.
hth!