While it appears to be worse for the IP4, prior phones seemed to have issues with the sensor going south when the phones were not actually subjected to water.There have also been many reports of iPhone sensors turning pink just because of humidity in the air. IMO, it's a flawed implementation. But it gives Apple a good excuse not to honor AppleCare.
I remember a lot reports of people using their phones during or after a workout and their sweaty hands or heads tripped the sensor.
While I can understand apple putting those sensors in (though I disagree with it), they should have have put them deeper into the phone and make them less sensitive to ambient humidity.