A few questions. Do you have an electric heater section (30KW) or gas furnace?
If you have a 4 ton, you should have a nominal 1600 CFM. Your supply opening off the unit should transition to something on the order of 20"x14" or 20"x12" main trunk line, for straight cool w/ electric heat and probably something on the order of 20"x16" for a true gas furnace. It really depends on what pressure your unit can work against.
What size unit did you replace..... 2.5 ton?
Sounds like you have a duct size problem.
Stepping down duct sizes for your existing duct system only add external static, more pressure drop for your system to overcome. Even your return duct of 8"x20" is to small if I understand you right.
Can you do a sketch of your existing duct system system with sizes ands lengths and post it? Include the manufacture and model number of your new unit.