If a Leblanc, Fragmented attacks into a battlefield with an enemy unit with assault and therefore is at 4 might, and I conquer it, on my legend trigger, is the Leblanc, Fragmented still 4 might and can the enemy gust the Leblanc away?

Your opponent cannot use Gust on LeBlanc, Fragmented because her Might remains above the targeting threshold during your Legend ability.

1. LeBlanc's Might During Combat and Conquer

• Assault Interaction: LeBlanc has the Assault keyword. Rule 807.1.c defines Assault as: "While I am an attacker, I have +X Might." As the attacker, LeBlanc benefits from her Assault bonus, bringing her total Might to 4.
• Duration: Per RiftJudge FAQ #8706, the might bonus from Assault persists until the combat showdown ends. This includes the period immediately after winning combat when you trigger your Legend ability to conquer. Therefore, LeBlanc remains at 4 Might during your Legend trigger.

2. Can the Opponent Use Gust?

• Gust Targeting: Gust targets a unit at a battlefield with 3 Might or less.
• Targeting Legality: Because LeBlanc, Fragmented is at 4 Might due to the active Assault bonus, she is not a legal target for Gust.

Note: I could not fully verify the printed base Might of LeBlanc, Fragmented against the retrieved rules.