I solo the group content with my alts. I wouldn't recommend doing content that's 5 levels or more below you. You'll get significantly reduced xp from that.
You don't get less xp from being 5 levels above a mission , it's just you won't level up as quick doing so. The xp stays the same until you are 6 and a half levels above the required , in which case you get 5 xp for completing said mission/s.
As to the OP , well it depends on which H2 , H4 or flashpoint you are talking about really. Say for instance Courscant , you can do The Face Merchants H2 in the black sun area , really at quite low level , now if you only use 1 character , then you will be done with the planet in next to no time, If you use a few characters on a daily basis then you could sit one character doing that mission once a day and get to level 18 ( it's a level 11 mission and at 18 you get 5 xp for completion ). Then you have the problem of finishing off your class quests. The justicars section actually still gives you xp after you reach 18 so maybe a mixture is good.
Another example would be tatooine , i actually sit my characters for about 2 weeks on that planet doing H2's on a daily basis , mind you I learned that leaving tatooine at level 35 might be a bit high as i then only gained just over a level and a half for doing the whole of Alderaan , did it without breaking a sweat though , which was nice.