There are 2 different ways in which you can eject a disc from the Playstation 4. The first and most obvious way is to press the eject button on the front of the console. The location may vary depending on the model, but since there are only 2 buttons on the PS4, it wont be that hard to figure out which one.
The other way to eject a disc from the PS4 is to use the controller and eject the disc using the option in the main menu.
By going to the game/disc item in the main menu of the PS4, you will have the option to eject the disc if you press the options button on the Dualshock 4 controller. Once you click this option the disc will eject from the ps4. The system will not completely spit the disc out so you will not need to worry about the disc falling on the floor.