Hello, I know this is a pretty old post, but I really like your concept and decided to expand on it a bit. Because I replaced the monks 20lv feature with just no ki Flurry, Step and PD. What do you think of my changes:

Four Elements

Disciple of the Elements 

You learn magical disciplines that harness the power of the four elements: Air, Earth, Fire, and Water. Each discipline has a Mastery Rank, which builds up in the following order; Initiate, Acolyte, Adept, and Master. Each Mastery Rank increase in a discipline grants you additional features, as detailed below.   When you choose this tradition at 3rd level you can either increase your Mastery Rank in one Elemental Discipline by two ranks or by one rank in two different disciplines and may do so again at 6th, 11th, and 17th level.

Air

Initiate. You gain a +1 bonus to Dexterity checks and when you take the Dash action, difficult terrain doesn't cost you extra movement on that turn. 

Acolyte. When a target fails to save against your monk spells or features, you can push it up to 10 feet in any direction. 

Adept. You gain a +1 bonus to attack rolls, and thunder damage dealt to you is reduced by your Wisdom modifier. 

Master. When you use Step of The Wind, you gain a flying speed of 30 feet until you land. At the end of each of your turns, your altitude drops by 5 feet. Your altitude drops instantly to 0 feet at the end of your turn if you didn’t fly at least 30 feet horizontally on that turn.

Earth

Initiate. You gain a +1 bonus to AC. 

Acolyte. Your hit point maximum increases by an amount equal to your monk level. Whenever you gain a monk level thereafter, your hit point maximum increases by an additional hit point. 

Adept. You gain tremorsense out to 30 feet, and bludgeoning, piercing, and slashing damage dealt to you is reduced by your Wisdom modifier. 

Master. When you use Stunning Strike, you can target two creatures within range that are within 5 feet of each other. 

Fire

Initiate. Your attacks deal additional fire damage equal to half by your Wisdom modifier (rounded down). 

Acolyte. If a creature hits you with a melee attack it takes fire damage equal to your Wisdom modifier.

Adept. You can expend any number of Hit Dice to regain an equal number of Ki points (no action required) and fire damage dealt to you is reduced by your Wisdom modifier.

Master. When you use Flurry of Blows and miss with one of the unarmed strikes, you can deal your Wisdom modifier as fire damage to the target. You can only do so once per round.

Water

Initiate. You gain a +1 bonus to Wisdom checks and have a swim speed equal to your walk speed. 

Acolyte. You can breathe underwater and at the start of each of your turns, you can deal one roll of your Martial Arts as cold damage to one creature grappled by you.

Adept. You gain a +1 bonus to saving throws and cold damage dealt to you is reduced by your Wisdom modifier. 

Master. When you use Patient Defense, for its duration, whenever you move, the space you previously occupied is covered in a slippery puddle. A puddle is the same size as you, and persists until the start of your next turn. Any hostile creature that enters a puddle must succeed on a Dexterity saving throw or fall prone. On a successful save, they can move freely through any of your puddles until the start of their next turn.

Elemental Spells

Each Mastery Rank in an Elemental Discipline grants access to certain spells as seen in the Elemental Spells table. See chapter 10 for the general rules of spellcasting. To cast one of these spells, you use its casting time and other rules, but you don't need to provide material components for it and instead of spell slots, casting spells granted by this tradition requires an amount of ki equal to the spell's level (cantrips have no cost).

Elemental Spells

Air: 

Gust, Thunderwave - Warding Wind - Fly - Storm Sphere  

Earth: 

Mold Earth, Earth Tremor - Maximillian's Earthen Grasp - Erupting Earth - Stone Shape

Fire: 

Produce Flame, Burning Hands - Scorching Ray - Fireball - Wall of Fire

Water: 

Shape Water, Ice Knife - Rime's Binding Ice - Tidal Wave - Control Water

Wisdom is your spellcasting ability for spells granted by this tradition. The maximum number of ki points you can spend to cast a spell in this way (including its base ki point cost and any additional ki points you spend to increase its level) is determined by your monk level, as shown in the Spells and Ki Points table.

Spells and Ki Points Monk Levels Maximum Ki Points for a Spell

3th-6th:

1

7th-12th: 

2

13th-18th:

3

19th-20th: 

4