Do you SEE your actors in battle? Either their images in the sideview battle, or their names in the battle status window? In the main menu, do you see them on the first page or do you have to page down to see them?
If you're seeing just the leader and nobody else unless you page down, it means you've added them incorrectly. You had 4 actors but did not want them all in your party, so you deleted them from the Actors tab, then you added new actors below those. Those "blank" actors are still in your party. The System tab is where you select who is in your starting party.
If you had it set up with "blank" actors in your starting party, started a game and saved it, those "blank" actors will always be there in that save file. Changes you make in the Actors tab will not affect that save file.
So if you make any changes to the Actors tab, or to the starting party in the System tab, then yes, you have to start a new game to see those changes.