What is the ruling if I recited the final Tashahhud after the first Tashahhud by mistake, while still having the third and fourth rak'ahs remaining: Should I say the tasleem and repeat the prayer, or should I rise for the third rak'ah?

If a worshipper recites the supplications of the last tashahhud in the wrong place, this action does not invalidate their prayer intentionally, nor does it necessitate the prostration of forgetfulness (sujud al-sahw). They should complete their prayer, and there is nothing upon them.