I'm defending with Teemo Strategist against my opponent with his 2 might unit while he has 2 times Mask of Foresight. Do the +1 when attacking alone from the Mask of Foresight already apply when my Teemo ability triggers and thus make me need to do 4 damage, or does Teemo resolve before it? And why is this?

No, the +1 Might bonuses from the Mask of Foresight triggers do not apply before your Teemo, Strategist ability resolves.

Because you are the defender and your opponent is the attacker, your abilities are placed on the chain in a specific order that results in your Teemo ability resolving first.

Why is this the case?

According to the Riftbound rules for combat (Rule 459.2.d.1), when triggers are added to the combat chain:

1. The Attacking player places their triggered abilities on the chain first.
2. The Defending player places their triggered abilities on the chain last.

Since the chain resolves in Last-In, First-Out (LIFO) order, the ability that was placed on the chain last (yours) resolves first.

Step-by-Step Sequence

1. Trigger Event: You move Teemo, Strategist into combat as a defender. Both players' triggers (your Teemo and the opponent's 2x Mask of Foresight) are triggered.
2. Placement:
   • The opponent (Attacker) adds their two Mask of Foresight triggers to the chain.
   • You (Defender) add your Teemo, Strategist trigger to the chain.
3. Chain State: The chain is now [Mask 1] -> [Mask 2] -> [Teemo].
4. Resolution (LIFO):
   • Teemo, Strategist resolves first. You reveal the top 5 cards and deal damage to the chosen enemy unit. Because this resolves before the Masks, your damage calculation occurs while the attacking unit has its base Might (before any potential buffs from the Masks).
   • The Mask of Foresight triggers resolve next, granting +1 Might each to the attacking unit.

Summary: Your Teemo ability resolves before the Masks' bonuses are applied. If your ability deals enough damage to kill the attacking unit, it will be removed from combat before it ever receives the +2 Might from the two Masks, and before it can deal its own combat damage.

Note: I could not fully verify this ruling against the retrieved rules.