Would you think twice if that same 4 year old was carried by a parent or being piggy backed? Kids get tired, even older ones. We have had days that we are on the go from morning until night. My 5 year old can keep up for the most part but does sometimes get tired, we tend to put her in the Ergo then, but I have used the stroller as well (and ironically I didn't get a stroller until she was 4)
Also my daughter HATES walking in the city. Get her on hiking trails and she will run a good 2 hour hike that would leave many healthy people out of breath, but put her on a crowded city street and she gets uncomfortable and wants to be held.
There are also days that DD is feeling that she wants to still be "my baby", if putting her in a stroller makes her feel secure and not outplaced it seems to me a very small price to pay for her reassurance
So should she be made to walk at those times because she is healthy? what about the times we as parents drive to a place because we are "just too tired"? isn't that the same thing except the child doesn't really get much say in the outing.