What should I do if I prayed four continuous rak'ahs before Dhuhr without saying the taslim after the first two rak'ahs?

It is better to pray the pre-Dhuhr regular sunnah prayer two rak'ahs at a time, but it is permissible to pray them four rak'ahs with a single taslim. If one intends two rak'ahs then stands for a third out of forgetfulness, they should return to the sitting position, recite the tashahhud, and perform the prostrations of forgetfulness (sujud al-sahw). If they intended to complete four rak'ahs after standing for the third, they should return to the sitting position, then stand with the intention of completing four, and then perform the prostrations of forgetfulness. Some say they can proceed with the intention of completing it without returning to the sitting position. If one does not return to the sitting position and completes four rak'ahs without intending an increase, there is no requirement for them to repeat the prayer because it is a nafilah (supererogatory prayer), though if they do repeat it, it is permissible. Imam Al-Nawawi holds that whoever intends a certain number of rak'ahs may increase or decrease them on condition that they change their intention beforehand. Otherwise, their prayer becomes invalid if the increase or decrease is intentional. If they stand up out of forgetfulness, they should return to the sitting position, recite the tashahhud, and perform the prostrations of forgetfulness. The Malikis hold that if one forgets the taslim after two rak'ahs of a nafilah prayer and stands for a third, they should return and perform the prostrations of forgetfulness. If they only remember after raising their head from the ruku' (bowing) of the third rak'ah, they should proceed and complete four rak'ahs and perform the prostrations of forgetfulness for the omission of the taslim.