Is it that I prayed Isha without the first Tashahhud, meaning four consecutive rak'ahs?

If someone forgets the first tashahhud and stands up for the third rak'ah, they should complete their prayer and perform the two prostrations of forgetfulness (sujood as-sahw) before the taslim. The ruling on the obligation or recommendation of sujood as-sahw in this case is a matter of scholarly disagreement. If one does not prostrate (for sujood as-sahw) until a long time has passed, then their prayer is invalid according to those who hold the view that the prostration is obligatory, and they must repeat the prayer.