What is the difference between the 3p sublevel and the 4p sublevel? on the periodic table

The p sublevels are named 2p, 3p, and 4p since the p sublevel appears only starting the 2nd level. By the Aufbau principle, 3p will be filled first before 4p. This is because of the energy present on the level. 

Each of the p sublevel has 3 orbitals, allowing them to contain 6 electrons as each orbital may hold two.