Must a 4 ton variable speed air handler use a 4 ton heat pump?
The question you ask about a "4-ton air-handler" is that you can expect the air-handler to produce enough air supply to support a 4 ton A/C unit requiring about 1600 cfm. The actual air flow is dependent upon the total resistance to air flow in the whole system. Since the motor supplied is a variable speed or more probably a 4 speed motor, then you could utilize a lower speed for a reduced air flow. The actual air flow needed to support an A/C unit is not an exact science but is an approximation of the air flow needed. So, to answer your question "yes", you can use that air-handler on an  A/C unit smaller than 4 ton. The 4-ton is the maximum size A/C you should use