I use Decree of Discord on two 2 might yellow units. In reaction, opponent uses Discipline to make one 4 might. What happens when my Decree of Discord resolves?

You return one of them, not both.

Here's why. Decree of Discord says "any number of enemy Order units with total Might 5 or less" — that's a group targeting restriction (Rule 355.11). When you played it, the two 2-Might units had a total of 4 (✓). Then Discipline resolves first (Reaction), making one unit 4 Might and the other 2 Might — now their total is 6, which exceeds the restriction.

Per Rule 355.11.b (with the Fox-Fire example which uses identical wording to your card), when the group no longer meets the total condition at resolution, you choose a legal subset of the original targets. Your options:

• Return the 4-Might unit alone (total 4 ≤ 5)
• Return the 2-Might unit alone (total 2 ≤ 5)
• But not both (total 6 > 5)

You can't pick units that weren't originally targeted, and you can't return both. Pick one.