I conquered with Sett with only 4 power and no buff on Sunken Temple. Can I use Sett's effect first to get a buff, become mighty, and use Sunken Temple's effect to draw 1 card if you're mighty?

No, you cannot use the effect of Sunken Temple in this scenario.

According to RiftJudge FAQ #4146, Sett cannot trigger Sunken Temple's ability if he is not already Mighty at the moment you conquer the battlefield.

Why this interaction does not work:

1. Trigger Condition: Sunken Temple's ability is a triggered ability that checks for a Mighty unit at the exact moment the conquer occurs. Because the "Mighty" condition is part of the trigger condition (it appears before the comma in the card text), the ability will not trigger at all if no unit is Mighty at that moment.
2. Timing: Sett's own "When I conquer" ability and Sunken Temple's ability are both triggered by the act of conquering. While you control both triggers and could normally choose their order on the chain, Sunken Temple's trigger condition must be met at the time of the conquer for it to be placed on the chain in the first place. Since Sett is not Mighty when the conquer happens, Sunken Temple does not trigger.
3. Buffing Sett: Even if you order Sett's trigger to resolve first to grant him a buff, it is too late to satisfy the condition for Sunken Temple, as the check for Mighty status has already failed.