I'm a lab chemist who takes measurements like these very seriously.
The trouble with temperature sensors built INTO a device (as opposed to connnected to the outside somehow) is that the equilibration time can be pretty long. It's also problematic to put the sensor in an enclosure along with a heat source (CPU, battery during charging, etc). As such, I'd only count on the thermometer being accurate if it'd sat away from your body, in the environment you wanted to measure for a while, screen off and not being charged.
Humidity sensing is notoriously difficult and slow even with lab-grade instruments that are periodically calibrated against salt-solution standards. I have very low expectations for the humidity sensor on the S4.
Pressure is easier to sense than temp or humidity. I'm cautiously optimistic that we can get good real time data out of that one.