To be fair, a couple of the 6 and 7 point episodes were actually 8 and 9 points but got a whole star deducted from them. So those episodes were actually a lot more memorable than it seems from the numbers. Personally I would probably have rated Season 4 higher than you as a whole; I thought the only below average episode was The Muse. (I am okay with the µ episode and would probably have given it 5 or 6)
Agree with you there, by TheGodBen's rating systems there were three of four episodes which were either 8 or 9 points but future continuity meant two points were deducted. I'll say this for season 4; it is an incredibly balanced season and one which is almost consistently very good. Season 5 may have a higher average but the ratings are more likely to go all over the place...