There might be smaller scripts that will let you do this without a bunch of extra stuff. I'm not sure.
Personally, though, I recommend checking out LegACy's awesome
1-Scene Menu System. Not only is it much better than the default menu, it comes with a lot of cool extra features like the ability to change the number of save slots the player has access to, a built-in party swapper, and an option to change the maximum number of members in your party. Just be sure to
read the directions carefully and if you have any problems, search through the thread, since LegACy's been very active in supporting the menu and has answered a lot of questions in there. If you want to use the icons feature, make sure you download the icons and put them in the proper folder. If not, you'll need to turn off the icon feature in the script before the menu will work.
Also, once you've changed the party max, you'll need to use events in-game to add additional characters. Even with the max changed in the script, you still can't start with more than four characters.