hmm thats a good question. In most games like that the interface is awful.

This is not an rpg but 'frozen synapse' and 'frozen endzone' are actually good examples of a UI for controlling multiple guys acting simulataneously.

Also I remember dungeon siege having a pretty basic interface but also being quite functional (with 8 people in you party!)

The 'eye of the beholder' / 'might and magic' / 'legend of grimrock' way of doing things is effective but it basically turns your 4 characters into one big one