Mountain Stance seems a bit concerning.
It seems like it's the support feat for Strength-based monk builds.
But then why does it provide a static +4 bonus instead of just letting you add your strength modifier? Ability boosts seem rather generous from the playtests, and if there are any magic items that offer stat boosts, it'll just make the gap even bigger.
Why is it a stance? If you're a strength based monk, should you really need to take a big hit to AC just for trying to use any of your other stances?
Where is the payoff? The example PC lowered their Dex and raised Strength just to take full advantage of the Mountain Stance...and for what? A dexterity based monk with 18 dex would have the same AC, and they'd deal more damage since they could switch to Dragon Stance without losing any AC.