Familiars don't advance to 4hd.
They have 1/2 the HP of their masters.
I could be wrong (am sure someone will be along shortly to say so, if I am) but Familiars don't get anything for their quasi-HD that the Familiar text doesn't say they get.
They don't get more skill points, feats, HP for con, or anything.
They get the skills their master gets, 1/2 of their masters HP, no feats or extras unless some ability says they get it.
-S