In 2nd stage, with the return air conditions you posted, the temperature drop through the system should be right about 20ºF, give or take a couple of degrees, for a system moving 400 cfm/ton.
According to your controller, the system is moving 350 cfm/ton, nominal airflow for a 4 ton system being 1600 cfm.
At that airflow, I'd expect the temperature drop to be more in the range of 20 to 22ºF with the return air conditions given.
Assuming all of the readings are correct, your system is operating way under capacity.
Has it been determined beyond any doubt that the compressor is actually running in 2nd stage?
The suction pressure is significantly higher than I'd expect to find in 2nd stage, it is more in line with what you would get with first stage compressor operation, and 2nd stage airflow.
The head pressure seems a tad low for the outdoor temperature, another indication that the compressor may not be operating in 2nd stage.
Is this one of the units with the Bristol TS compressor, or does it have the Copeland 2 step compressor?
The Infinity system is capable of giving you a temp drop of approximately 27 degrees when operating in comfort mode. It seems high but I've observed in on multiple occasions. The 1400cfm indicates it is operating in comfort mode. (350cfm per ton).
360psi indicates a 107-108 p/t. With a 96 deg liquid line temp the subcooling = 11-12 deg.
The report stated 15 degrees.
You may be a little low on refrigerant since the data plate specifies 15 degrees.
I'm not trusting his numbers. I've taken measurements at different points at the duct work and had a large variation. Eddies in the airflow near the evaporator coil can create cold spots. I've found checking supply air further downstream will sometimes point out a cold spot and thus an erroneous supply temp.
I'd recheck the return temperature at the furnace and at the return grill. You could be drawing in air from leaking return duct. The relative humidity drop of only 2%? Hmm,
A leak in the supply duct work can dump air outside of the conditioned space. To make up for the air loss the building will draw outside air into the structure. This will increase the load on the system and your ac will not hold setpoint on hotter days.
This could explain a relative humidity drop of only 2 degrees.
I'm not a Carrier guy, so don't know by the model numbers what they came with.
The only 2 Infinity systems I've delt with were well after the customer gave up on the installing contractor. In both cases it was just minor stupid installation/setup details that were giving the system fits.
There are also a range of favorable conditions under which that SC value is valid. Instructions are to weigh in the charge when conditions aren't favorable and come back later when they ARE favorable to trim the charge.
Oops, just relaized this is in the AOP forum. I hope this doesn't qualify as DIY info.