The only method I know to activate the debug mode without hacking the game(or its RAM) requires that you have expansion ROM/RAM with specific values at bank $FC, and that you hold I+RUN during startup until you get this screen:
Here's a cheat code that very-partially simulates the presence of this (presumed) development device, if you're curious:
[6350e1c06f4d5be0a27eb0ae31c604fb] Ys IV - The Dawn of Ys {HCD3051-4-1108-R1F} (SCD)(JPN)
S A 8 L 0 001F9200 e7366d6f72204244 Simulate ROM necessary to allow for debug mode.
...though doing it this way seems to prevent you from accessing the dev/staff room from the menu under "Continue" like you can with the updated simple cheat code; if you try to select it, you'll be sent to the NPC debug village instead.
...but of course I can't ever rule out the possibility of there being a more player-friendly way to activate the debug mode, as I've only examined a tiny fraction of Ys 4's code.