You can also try this macro:
In the past, I have used it to merge hundreds of files and it worked really well.
Give it a try and start with the same 4 files you have used.
Maybe you can also try moving them to a different location? Maybe it is working slowly because they are located in a location that is slowing everything down. I remember when I used Alteryx and box it wasn't working efficiently together.
Maybe check if moving the files to your desktop would speed up the process.