[Scripting Question] Region-Locking Pokemon and Items

I would like to have a game with four separate parties, PC Boxes, and inventories. How do I accomplish this?

so, you have multiple regions and want the player to "start over" in every region?
RPG maker XP allows multiple save files, but Essentials forces a single save-file.
if you edit that, you can create a new save file, copy only the player's name into it, and place the player in a new region's starting map. you'll need a script for this. then,by loading the first save file, the player can travel back to the first region, and have all their stuff still there.

This tutorial will give you some insights:

https://pokemonessentials.wikia.com/wiki/Tutorial:Multiple_Protagonists