I don't know if anyone ever gave information on save file editing and on this case in particular; if the search function in this forum section (DOS - Mods) doesn't reveal anything (save file editor for DOS 1, cheat engine, whatever), the only way is to search through the save file for hints, though it's not unlikely that the data in question is not readable text but a number or something.
The database name in human-readable story code is "DB_CompMax".
I don't remember the global flag's name (not a dialog character flag as I wrote above) and I don't have the game installed at the moment, but it shouldn't be strictly necessary to change the flag if it exists (in case the party was full in your current save, dismissing a companion should remove the flag).
Though, I would just restart, if you're not far into the game, or finish the playthrough with 4 characters and use the mod for a second playthrough (maybe in addition to other, compatible, mods or as part of another mod to make it more interesting). Otherwise you could ask on the official Larian Discord for help, where the modding community has moved to years ago (some of them may be/are familiar with DOS 1 EE).
Last edited by Abraxas*; 25/09/21 08:23 AM.