I do not know what the four flash code means as the codes vary per brand. The pressure switch just confirms that the inducer is venting properly. Just because the flash code (if it said pressure switch) says pressure switch it doesn't mean the switch is bad, just that it is why it won't let the furnace light. So, it could be bad (which seems it wasn't since you replaced it) or it could be a blockage in the chimney, the motor is bad or going bad, and a couple other less likely possibilities. Electrical testing the circuit would help if you know how to use a voltmeter and run these tests to help confirm what exactly the problem is.