You need to be a citizen for at least 7 years to be eligible to hold office, but Santos may have just lied on the paperwork, and the people in charge of checking that shit have little incentive to actually check, because Santos is a Republican who blindly supported the agenda, until he became a liability to the party.
The medical science on it says that two years of hormone therapy are enough to lose any sort of competitive advantage in trans athletes.
Four years is overkill, and is part of why this policy is considered one of the strictest in professional sports.