Is it valid for the questioner to fulfill her vow, after she vowed nine years ago to pray four rak'ahs daily, and she only prays two rak'ahs?

There is no two-rak'ah prayer called "Salat al-Shukr" (Prayer of Gratitude) in Islamic law. Rather, there is the prostration of gratitude (Sujud al-Shukr), which is recommended by the majority of scholars upon receiving a blessing or repelling an affliction. Whoever vows to perform it must fulfill their vow; if unable, they must offer an expiation for breaking an oath. As for Salat al-Hajah (Prayer of Need), it is legislated, and a vow to perform it must be fulfilled. The prayer performed on the last Friday of Ramadan with the intention of making up missed prayers is a forbidden innovation, as are Salat al-Ragha'ib and Salat al-Bara'ah, because their narrations are fabricated and because they have become a cause for the common people to neglect their obligatory duties.