What you need to understand is that serato and Engine Desktop are not the same type of software, they do not have the same approach, nor the same philosophy, and they are not made to do the same thing.
Serato is live mixing and performance software for mixing in real time, which you control for example with a controller.
Engine Desktop (which should not be confused with Engine OS which is the software integrated into the hardware) is library management and track preparation software.
Engine Desktop has no other purpose than to help you organize your playlists, create them, add or remove tracks, analyze the tracks to define the grid, place point cues on these tracks, etc.
You can do most of these actions directly on the hardware, just it’s quicker and easier to do it on Engine Desktop, then at the end export everything to either a USB stick or external storage media.
Watch the different tutorials on Engine concerning the creation of playlists, library management, positioning of cue points, and the export process to your storage media.
Once this work is done you will be able to enjoy your music more easily on your equipment. You can obviously do this work directly on your hardware and then synchronize what you have done with Engine Desktop to recover your hot cues or the playlists that you have created on your hardware. It works both ways.
You can completely do without Engine Desktop and simply place your music files on a storage medium and access them from the folder menu of your SC live 4. But the tracks will take longer to be loaded and analyzed because it will not have been pre-analyzed by Engine Desktop, which is much faster for analyzing tens and hundreds of tracks in batches. And you will have to create your playlist manually from your hardware which can be tedious.
But keep in mind that Engine Desktop is not performance software, designed to be driven by your Denon equipment and for live mixing. It is just designed to prepare what you will do later live directly on your Denon equipment independently, without a computer.
You can nevertheless use your SC live 4 to control serato if you prefer, but you lose the benefit of the autonomous side of your equipment and you remain dependent on a computer.