Like many others said, it depends on the circumstances. My little girl is 3 years and five months old and hasn't been in her buggy more than twice since she was two, but she actually likes walking a lot and can walk for quite a fair distance.
Generally I don't entirely agree with using a buggy for a 4 year old if it is just for a ten minute walk or so, because the exercise is good for them, but for a walk of an hour or so, if you feel that it is too far for your LO to walk then why not??
Also, like someone else said, it takes a lot longer to walk with a child, so that one hour walk could end up being a two hour walk without the buggy. It's sort of like an adult driving to the local shops rather than walking... sure, they could walk.. but it is quicker and more convenient to drive sometimes. Don't worry about what others think- it is your child and you know what they are capable of. xx