abirdcall
(she/her)
Yeah, this.
I ran a 2-year campaign where one of the PCs was a Sun Soul monk, and it became clear that Stunning Strike was far and away the best use of her ki. Get up to the biggest, scariest monster, blitz it with unarmed strikes, and toss a Stunning Strike on each one until the monster blows a save. Not only does the entire party get advantage to hit, but you effectively "heal" everyone for all the damage the monster would have laid down on its turn. It was infinitely more effective than any of her "spend a bunch of ki for a little AoE damage" abilities.
As a result, while the character certainly pulled her weight in combat, she became something of a one-trick pony. It looks like the Four Elements monk has the same issue, and upping its ki pool doesn't do anything to change this dynamic. The monk will still spam Stunning Strike; it will just go on longer.
I'd focus instead on bringing down the cost of elemental abilities. Or supply a dedicated "elemental ki" pool which can only be used for those abilities.
The Sun Soul gives the Monk better options in some fights.
The regular Monk excels at neutralizing big threats, hopefully with low Con. Enemy wizards are a great target.
Not every combat encounter has those creatures however. In that case Burning Hands is a better use for their Ki.
By the same regard their ranged attacks won't be useful in most combats, but every now and then they will. Radiant is the best damage type and fairly hard to come by so that is nice. The ranged part is decent, and overall better than a shortbow as they can flurry of blows with it (and they're magical radiant damage instead of non-magical piercing). Magic strikes starting at level 3 is also quite good as this is the time the party will have trouble fighting creatures that have resistance to non-magical attacks (and those creatures might also be weak to Radiant damage). Level 5 takes a long time to get through so having magic before 6 is good.
The Sun Soul is on the weaker end of the Monk subclasses, probably around the Drunken Master. The Sun Soul has abilities that are good, they're just narrow in use.
I don't think anything is going to save the 4 Elements Monk. I think the better way to approach it is to use the Sun Soul as a template. Pick an element and create abilities that incorporate the Monk framework. It's not just that it is very weak, the design of it is a mess as well. It is an attempt to make a 1/3 caster like the Eldritch Knight and Arcane Trickster, only instead of getting spell slots, their spells eat into their Ki.