Probably the simplest way to impose an existing multi-page PDF to a new PDF showing 4 pages per sheet (e.g. 4-UP) is to install and use the free command-line tool named cpdf from Coherent PDF. When you click the link to download the pre-built cpdf binary tool, it will download a file named cpdf-binaries-master.zip into your Downloads folder. In the Finder, you double-click that file to unzip and that leaves you with a new folder named cpdf-binaries-master. In that folder is a series of folders for platform binaries. If you are on an Intel Mac, you enter the OSX-Intel folder and copy the cpdf binary to your /usr/local/bin folder, or the OSX-ARM folder if on an Apple Silicon Mac.
The current cpdfmanual.pdf is in the cpdf-binaries-master folder. It is detailed with plenty of examples.
After you have installed the cpdf binary in /usr/local/bin, you can click this link to install the Apple Shortcut that creates a new 4-UP PDF from an input multi-page PDF. The output PDF should just use the same paper size as th input, though you can override that by using page names from section 3.1 of the manual.
I have tested the following Apple Shortcut on Sequoia v15.5 with cpdf v2.8.1 and it works well to produce a 4-UP PDF with the filename suffix "_4up" in the same folder as the selected PDF or PDFs. Once the Shortcut is installed you right-click on a selected PDF(s) in the Finder and select Quick Actions > PDF to New 4-UP PDF.

Here is an example result:
