Getting the disk version of a downloaded game

No, there's no way to do that.

But you can safely delete games from your PS4 and re-download them at a later time. Digitally purchased games are associated with your profile and you still "own" it even if you don't have it installed. This can make room for more digital games.

Also note that even if you have a disc copy of a game, it has to "install" to the PS4, which functionally is the same as having the digital copy with the added hassle of needing the disc in the system to play. Essentially, there is no advantage at all to having a disc copy unless you like collecting physical objects for their own sake.