Depending on what pic, and what pin on that pic you use... Some pic have 5V tolerant input on some pin. So check that in datasheet. Most of pin on pic have protective diode. So if you apply 4,2V, you supply voltage may increase to 3,5V.
You can just put resistor(10K) on on input pin, to prevent this.
Hi an Lo state of pin depending what type input it is... Eg ST, CMOS, etc... Value for each input type is in datasheet.