A 4 ton will freeze up on a duct system designed to handle 1000 cfm or less, or at best perform really inefficiently.
The sizing you've gotten from these contractors is beyond absurd unless you're in a brutal climate like phoenix, Arizona.
1 ton per 500 sq ft will get u into big trouble and it's BS dating back pre-1980 from when building science wasn't even a field and electricity was under 5 cents per kwh delivered all inclusive.
Where are you located?
2.5 tons is not at all out of line for a 2000 sq ft house at least from what I've experienced with ducts in the basement and decent insulation. Okay, you may need a bit more capacity with ducts in the attic, may need a bit more in a climate which experiences 95f+ more than a couple of days each year. Don't know about the construction either.
Wouldn't get more than 3 tons without an accurate heat loss/gain calc as well as major changes to the ducts. (which could be too small for what u have)
But the symptom of a marginally sized but well performing a/c is running continuously, having the temperature inside rise a few degrees above setpoint in extreme weather but otherwise cooling well and pulling out loads of humidity. (...and if it's pulling out loads of humidity having it drift up isn't such a big deal)
Bigger unit doesn't dehumidify as well and stresses the electricity grid more -> meaning forces the utility to build more peaker power plants, jack up the rate to cover for that.