I find that four characters is not much. Six was a good number. There are probably good reasons for reducing the number to four. But instead of putting the companions in reserve at the camp, why not allow them to be sent on a mission somewhere? - Search for information - Escort missions - Surveillance - Walking around and scouting - Shopping Maybe even form a second group in parallel.
The old SWTOR party crafting thing basically. That definately sounds like a cool idea.