Why is application switching slow on Late 2013 Mac Mini compared to early 2008 Macbook Pro?
4GB of RAM isn't much to be running all of those heavyweight apps at the same time. My guess would be that it's doing a lot of paging to disk. Check the size of the swap files on both machines.


Go to the Finder's "Go" menu and select "Go to Folder…".
Enter /private/var/vm/ into the box and press the "OK" button.
Add up the sizes of the swap files. 


If it's more than 1GB I usually reboot. That will clear out these files. I've seen these files up to 4GB on a machine with 8GB of RAM.

Since running out of RAM can cause writes to disk, the difference in disk speed may be what's causing the delay.