If I use En Garde on Irelia Fervent while she is alone at a battlefield, does she get +3 might or +4 might?

Irelia, Fervent will receive a total of +3 Might when you play En Garde on her while she is the only unit you control at that battlefield.

According to RiftJudge FAQ #9564, the sequence is as follows:

1. Targeting: When you play En Garde and choose Irelia, Fervent as the target, her own ability triggers because she has been "chosen." This grants her +1 Might.
2. Resolution: En Garde resolves. Because she is the only unit you control at that location, it grants her its base +1 Might plus the additional +1 Might bonus, for a total of +2 Might from the spell.
3. Total: 1 (from her own ability) + 2 (from En Garde) = +3 Might.