Output of 4.8V due to reverse polarity series schottky diode?
Here's the relevant part of the system diagram from the datasheet you linked:



A Schottky diode typically has a forward voltage drop of 0.2 to 0.3 V. So if 5 V is applied at "V+", then only about 4.8 V is available to supply the Richtek switching converter or any other circuits that might be connected to that same circuit node.

The reason it's called a "reverse polarity protection" diode is because its function is to protect the circuit in case the USB connector is somehow connected in reverse, applying ground to the "V+" node and +5 V to the ground node of the circuit. In that case the diode will be reverse biased and not allow current to flow through the circuit.