I would like to find out the normal operating range of pressure (high and low side) on a fully charged R-22 (york) home air conditioning system; also, the unit is a 4 ton unit, what is the total freon capacity in the system. How much does it hold?

Pressures and refrigerant capacity will vary depending on first how old the equipment is (the older the system the more they might hold ) the older systems also ran a higher head pressure as well, the pressures also varied as to what metering device is installed. Look on the name tag for system refrigerant capacity.
The suction ptressure will range from 60 to 80 lbs. and the head will range from 170 to 325 lbs. The charge is based on sub cooling or superheat and that's the best way to charge a system. Good luck . Mike

In addition to the info provided by acetc, other factors will impact operating pressures. Insufficient indoor airflow (clogged air filter, clogged evaporator coil, blower not operating, etc.) will typically result in lower than normal pressures. Insufficient outdoor airflow (clogged condenser coil, fan not operating) will cause higher than normal pressures. Assure that you do not have problems in these areas before evaluating system performance.

Do you about Freon 22a and if you can drop it in where r22 was

do you about Freon 22a and if you can drop it in where r22 was

R22a is highly flammable & should not be used, period. Here is a link to EPA's website which discusses R22a. Alternatives / SNAP | US EPA (http://www.epa.gov/ozone/snap/r22a.html)