What should you do if you realize just before saying salaam that you prayed one less rakah in salah?

So if you feel like you're one rakah less, what you would do is immediately stand up and start reciting Surah Fatiha. So that would be your fourth rakah essentially. And then when you get to the end of the salah, you then do Sajdah al-Sahw. There are two ways in which you can do Sajdah al-Sahw, the prostration of forgetfulness. One way, and both are recorded in Sahih Bukhari, is that you sit down and you recite all the way until the end as you normally would do, then any duas you normally do, and then you say, do two extra sajdahs, come back up and then say. That's one way. The other way also in Sahih Bukhari is that you recite just until the end of the Tashahhud, then you say to the right, and then you do the two extra sajdahs, then you come back up and you start again from to the end, and then any duas you recite normally, and then as-salamu alaykum wa rahmatullah, as-salamu alaykum wa rahmatullah, as-salamu alaykum wa rahmatullah. Both views are valid.