Four years ago when I bought my house, I feel the cold air comes out from the vent really strong when my AC is on; but now the cold air comes out very weak. Last week my AC was charged with 4 lbs of freon type 410 and I have changed the air filter; but it the cold air still comes out weak.

Did the tech look at the blower and blower motor to see if it was working properly? Other than that there must be some sort of blockage in the duct work somewhere.

More than likely the evaporator coil is dirty on the return side. Seen this to be the cause many many times

4 pounds of 410??

Why?
What was your superheat?
What was your subcooling?
What were your suction and discharge pressures?