As already pointed out by some others, using a Companion Character (DMG2) is usually the best way to go.
Trust me, even at paragon tier, making and using a Companion Character is easy. And in my experience, making a useful Companion Leader is relatively easier than making companion characters of other roles. They are most stable.
Well, maybe aside from Ardent (who don't has encounter powers and thus not supported by current Companion Character rule), and Shaman (which relies largely on Call Spirit Companion, which is a class feature), all the other leaders work well as a companion character.
A companion PC is somewhat inferior to PCs. In case of Companion Leaders, they don't heal additional HPs when letting someone spend a healing surge and such. But they have at least as much HPs and healing surges. And their non-AC-defenses are basically quite good (level +13 for all the NADs). So they are relatively tough.
And, if you still want to let 4 PCs adventure only by themselves, how about giving them, say, Divine Boon Pelor's Sun Blessing (DMG2)? If all the 4 members have this boon, they each can let an ally spend a healing surge once/day. L13+ versions even have some bonus HPs regained.