.. You have already got some good comments! As pointed out, the answers depend on the actual torque development over time necessary for loosening a closed valve from the seat for minimum dimensioning, and physical possibillities and limitations (dimension and adaption parts, and max for instance stem torque) for maximum dimension.
As, even under 'good operating conditions' the necessary valve torque can increase considerably over time, dimensioning actuators too low is the normal common mistake.
Using actuators dimensioned for larger valves may be economical, but will often require additional parts, and require more work at mounting and adjustment.
An idea might be to check if the actuator selected for one size could easily be utilized for the size below.
In my experience the smallest actuator in a complete series fabricated could very often be forgotten, and the next in size used.
But: please be remained that for cheap butterfly valves the best and cheapest way of repairing could be to stock complete valves with actuators mounted, ready tested and adjusted. (Note: not to be stocked with valve closed! Will deform valve sealings.)
Only under certain conditions you could expect pneumatic actuators of smaller size ( 4" valve is small) to be worthwhile the cost and effort to change inner sealings or parts, testing and to be expecting a renewed full lifetime for the actuator. Cost would often be more than 50% of new actuator price.
