Its definitely very underpowered. I feel that a spell with 1 hr duration should not require concentration, especially one with an effect as weak as this. Casting Stoneskin means you can forget about casting any other spell that also requires concentration, lest you dont mind it ending early. Obviously you would pre-cast this spell if you foresee a bunch of tough fights ahead, but if in the first fight your best option is to Banish a very dangerous baddie, its a wasted 4th level slot there.
Casting it on yourself is weird, because you should be trying to avoid taking damage, and when you do, you might drop concentration, and the chances are not in your favour. I mean come on, by the time you get this spell, every monster and their mother have multi-attack. Casting on others is meh, when other buffs like Haste or Freedom of Movement is better. And speaking of Freedom of Movement, that's how a level 4 single target buff should be. Nobody said it was overpowered just because it didn't require concentration.
Its just an un-fun spell no matter how you look at it. Defensive spells are already less fun than offensive spells in general, but they somehow have to make this one really annoying to use by adding concentration and component cost.