I seem to recall Eidolons getting 5.5 per hit die, on a d10, but that may have been with an evolution.
Yeah, my bad. Five hit points, not four. Still not a great load of hit points for a melee combatant.
The Druid's Companion gets 2 HD at 1st level, so it's not as painful that they aren't PCs and don't benefit from any 'max hp at 1st level' rules. (And the ability to wear leather barding (no ACP) or MW studded leather (still no ACP) or mithral chain shirt barding (look ma, no ACP!) without having to blow a feat on light armor proficiency, helps the Druid's companion offset some of it's AC woes.