If a might 4 unit with master yi legend are defending a might 2. And get switcheeroo, does the +2 from defending alone still applied last?

Yes — but the +2 is already baked into the swap, so your defender ends at 2 Might, not 4, and the attacker jumps to 6.

Switcheroo snapshots each unit's current Might at resolution — your defender is 6 (4 base + 2 from Master Yi) and the attacker is 2 — then applies a fixed −4 to the defender and +4 to the attacker (Rule 433.1.b). Master Yi's +2 is a continuous passive that keeps applying while the unit defends alone, but since it was part of the value Switcheroo swapped, you don't get it added on top again: 4 + 2 − 4 = 2 for the defender, 2 + 4 = 6 for the attacker (RiftJudge FAQ #341 — passives stay on their original units and don't re-stack over the swap).