For years, I mistakenly prayed my Salah by reciting Surah Al-Fatiha and Surah Al-Ikhlas in the 4th Rakah instead of Subhanallah walhamdulillah. Does this mean my prayers are invalid, and do I need to redo them?
Your Prayers are valid and you don't need to redo them because you did not know the rule. No onward, you need to recite in the third and forth Rak'at either one time Sura Al-Hamd or Tasbeehat Arab's three times., then one time ASTAGHFIRULLAH.
Wassalam.