How can one make up for a forgotten prostration (sujud) in the first rak'ah when remembering it in the fourth rak'ah?

If someone forgets a prostration (sajdah) in the first unit (rak'ah) and does not remember it until they begin reciting in the subsequent rak'ah, the first rak'ah is invalidated, and the second rak'ah becomes the first. Similarly, if they forget a prostration in the second rak'ah and do not remember it until they begin reciting for the third, the second rak'ah is invalidated, and the third takes its place, with the fourth taking the place of the third. In this case, they must perform an additional fourth rak'ah to replace the invalidated first rak'ah.