I'd just like to point out that the devs said "a group is 4 players but COMPANIONS count against this total". I believe it even comes up as a tip on the loading screens sometimes.
ALL Heroic 2's should be soloable with your companion AT LEVEL (depending on class this currently isn't always true).
Heroic 4's should be doable with just 2 people with their companions, this isn't true either for all of them. Although as mentioned some are able to be done solo.
Honestly some of the solo class story fights are harder than some of those H2's. And therin lies the problem, there is absolutely zero consistency throughout this game.
BTW, I was doing this as well, because I was bored, until I found out the rewards are pretty much crap. It takes alot of effort and WAY more time to do these and you basically get nothing useful out of it. Even the XP being double makes it less efficient. It takes way longer then twice the time to drop a group of elites or champ and therefore you are actually wasting time. And to get nothing to drop but a couple credits, often less than what i get from a group of 4 normal mobs... Well that just kinda sucks. And the actaul quest rewards 9/10 aren't worth the time either, not even an upgrade in most cases just more vendor trash.
I as a tank and My brother as a healer have Done every single heroic +4 with our 2 companions as dps. So you can do them with just 2 people and 2 companions. obviously we kept up on our gear and our companions gear. and we where Assassin tank and Operative healer. So we had 2 saps and a droid cc pretty much it most the time i just aoe tanked everything and said f cc.
the quest reward xp is only half of what your earning by doing htem. The heroic content xp is per kill is higher then normal quest xp per kill. Normaly doing 1 heroic +4 and doing hte bonuses will yeild you close to a full level.