I don't have a definitive answer to your question. But I just read a review of the most expensive configuration of the ProBook 450 G4, and it didn't have a backlit keyboard. I would assume that none of them probably include one if the most expensive model doesn't. So my best guess is that the 450 G4 series doesn't support backlit keyboards.
EDIT:
After further investigation, the service manual parts list shows both backlit and non backlit keyboards. So it might be worth a try:
http
/h10032.www1.hp.com/ctg/Manual/c05270979
Under "Product Description" on page 4, both types are mentioned. And on page 13, a description of a particular hotkey being pressed in conjunction with the Fn key toggles the backlight on and off. Parts list is on...