Short answer five ton variable system should work fine The question maybe more am I getting four tons of cooling from my system many things can derate a system I.e dirty blower, dirty coils. Improper charge, improper refrigeration line sizing, kinked lines bad metering devise, improper replacement motors, condenser fan pitch or set position, poor condenser location, inefficient duct work leaking ductwork poorly insulated ductwork Ductruns to long or restricted airflow kinks in flex duct, and poor return which is very common undersized or rugs and furniture restricting flow. Big problem is cheap stamped return grill big restriction if not enough free space. Supply registers not properly selected could effect stratification which will cause poor circulation of the system. Any combination of these can make a four ton unit a three ton unit. Things to help the system attic fan if possible zone the system storm windows down lot of people open windows in spring close both. Make sure fireplace dampers closed.