The cantrip is almost completely irrelevant to discussing whether stoneskin is "worth" a 4th level spell, as the cantrip can't be used on anyone but the caster and "until the end of your next turn" isn't even in the same league, let alone ballpark, as "concentration, up to 1 hour."
Sure, when used the way people are accustomed to using stoneskin that doesn't seem like much - but when you actually embrace the new paradigm, you can squeeze a whole bunch of effective HP out of that 1 hour duration... it just might not be the most exciting thing for the caster to do, though, since it hinges upon the caster putting the spell on someone that is going to be taking repeated attacks of the right sort and then doing their best to avoid being the target of any attacks themselves.
At 100gp per pop, Concentration and any sort of magic weapons circumventing it, I'm not sure it's worth a 4th level slot. At the very least it's useless for NPCs.
I'm half-tempted to change it back to Adamantine weapons are required to punch through, like in 3.x/PF. (Though I realize Adamantine weapons aren't even really listed as an option in 5E. At least not in the DMG.)
Currently, Stoneskin has the following restrictions:
1. Magical weapon damage bypasses resistance.
2. Concentration
3. 100gp cost.
I'd suggest removing one of them. I'm leaning towards the first. Blade Ward, unless it's been errata'd, actually just plain resists the damage, magical weapons do not bypass it. (Spells, however, do, as it says "bludgeoning, piercing and slashing damage dealt by weapon attacks")
I have yet to see a use for stoneskin. It gives resistance to nonmagical bludgeoning, slashing and piercing damage and requires concentration. There are better buffs to cast on allies and casting it on yourself doesn't seem like a great use of a concentration spell.
Last night I had a caster villain that had cast polymorph on a minion (giant ape) and a PC had been turned into a t-rex.
Eventually the t-rex player got the brilliant idea to go chomp on the caster and breaking his con with 30 plus damage. It would have been a great use of stoneskin to have cast it on himself and cut that damage down.
Are others finding it lacking? Or am I missing something?
I think there is a divider among opinions on stoneskin that hasn't quite been expressly mentioned yet: whether the spell is being considered for use by the PCs against monsters, or whether the spell is being considered for use by monsters against PCs.Currently, Stoneskin has the following restrictions:
1. Magical weapon damage bypasses resistance.
2. Concentration
3. 100gp cost.
I'd suggest removing one of them. I'm leaning towards the first. Blade Ward, unless it's been errata'd, actually just plain resists the damage, magical weapons do not bypass it. (Spells, however, do, as it says "bludgeoning, piercing and slashing damage dealt by weapon attacks")