4" casings with a pitless adaptor are possible, but are relatively rare. Usually the top section will have been made 6". This makes the installation of a pitless much easier.
If the well is 4" diameter all the way up and with a pitless, installation might be possible with a bit of maneuvering. If the pitless doesn't allow for the installation of the Simple Pump, the pitless adapter can always be replaced by a slimmer kind of pitless adaptor.