وَإِنْ قَعَدَ فِي الرَّابِعَةِ، ثُمَّ قَامَ، وَلَمْ يُسَلِّمْ يَظُنُّهَا الْقَعْدَةَ الْأُولَى: عَادَ إِلَى الْقُعُودِ مَا لَمْ يَسْجُدْ لِلْخَامِسَةِ، وَسَلَّمَ، وَسَجَدَ لِلسَّهْوِ.
Translation: If one sat after the fourth, then rose without salam thinking it was the first sitting, one returns before prostrating in the fifth, says salam and performs sujud al-sahw.
The obligatory final sitting after the fourth was already done. The error here is delayed salam and unintended extra standing, rather than omission of the fard sitting.
Returning before the fifth’s prostration preserves the completed fard prayer, and sujud al-sahw compensates for the addition.