I have a page (actually more pages) on which there are 4 slides. How could I split the 4 slides in order to have a slide on each page?
You can choose whether to split horizontally or vertically and adjust the exact position of the split line.
By doing this twice, you should be able to get a resulting PDF that has a single slide per page.
To ensure the correct slide ordering in the resulting PDF you should first split vertically and then horizontally, in case the slides are ordered left-right and top-down. (The opposite approach should be taken if the slides are ordered top-down and then left-right in the original PDF).
If you however still need to reorder the slides after the two split operations, you can use Sejda's reorder PDF pages
You don't state what you want to do, other than to split them. As such, we have no idea if you need to end with PDF "slides" or not. We also don't know if the originals are PDF or images saved as a PDF.
Since you've simply asked how to split them, then one of the easiest ways is to zoom in to your PDF until the size is as you want, and then use a take a screen shot. In Windows 10, the snipping tool is perfect for this.
To create the PDF, there are many free online tools which can convert from image to PDF
Alternatively, open the PDF in Photoshop or similar and cut it up and export it as you wish.