I believe a cart isn't enough, because it only disables the region check in software when booting a foreign-region game, whereas a switch actually puts the Saturn into a given region the moment you turn it on. Say if you have a US Saturn and play a Japanese game with the AR cart, it'll still run on a US Saturn (as far as the Saturn itself is concerned).
I have a Hitachi Saturn with the Hitachi VCD card. I took the Hitachi VCD card out of my Hitachi Saturn and put it into my modded US Saturn. I plug in my Action Replay 4M Plus cart to my modded US Saturn and try to play a VCD. It doesn't work, says something about "Application Error", IIRC.
I RECANT!!! I RECANT!!! I got it my Hitachi VCD card to play VCDs on my modded U.S. Saturn with the AR cart4M+ cart plugged in!
Here's what I did:
[*]plugged VCD card in at the back
[*]plugged AR4M+ cart into the cart slot
[*]put a VCD in, closed the door, and powered on
[*]Pressed A on "START GAME"
[*]Pressed C on "START GAME WITH NO CHEATS ENABLED"
I realized out what I did wrong in my previous attempts. Before, I pressed A instead of C on "START GAME WITH NO CHEATS ENABLED" which gave me the "...LOADING..." screen and it just stood like that forever which lead me to believe that my beloved Hitachi VCD card didn't work on my modded Saturn even with the AR4M+ cart. Pressing the C button did the trick. Silly me! Thanks mal for pointing out that the C button takes you back to the CD menu which then takes you to the VCD menu! I should have read the details and tried that option back then.
FYI, before entering the VCD menu, it shows a blue screen with this info at the bottom half: