If I equip Brutalizer and then re-equip it using Jax's legend ability, do I get +2 or +4 might?

You do not get +4 Might. When you re-equip Brutalizer using Jax's legend ability, your unit will have a total of +3 Might from the Brutalizer (its base +1 Might and its conditional +2 Might).

According to RiftJudge FAQ #8566:

1. Detachment: When Brutalizer detaches from your unit to be re-equipped, the unit immediately loses all bonuses provided by that equipment, including the conditional +2 Might. The unit returns to its base Might.
2. Re-attachment: When Brutalizer re-attaches, it provides its base +1 Might and its conditional +2 Might again (because it was attached to the unit this turn).

The bonuses do not stack or persist; they are removed upon detachment and reapplied upon re-attachment. As stated in Rule 136.3.a, "The Might Bonus is applied while Attached and stops applying as soon as the card with the Might Bonus is no longer Attached."