What should a person who does not sit after the third rak'ah of the evening prayer and stands up for the fourth rak'ah do?

Should he sit down again and perform prostration of forgetfulness (sajdah as-sahw) or perform another rak'ah after that rak'ah and perform prostration of forgetfulness?

Dear Brother / Sister,

If a person stands up without sitting after the third rak'ah of the evening prayer, he needs to sit again if he has not completed the rak'ah with standing, qira'ah, ruku and prostrations. Then, he needs to salute after reading at-tahiyyat and perform prostration of forgetfulness. For, in that case, the last sitting, which is fard, has been postponed. However, if he prostrates for the fourth rak'ah, this prayers becomes a supererogatory prayer. Therefore, prostration of forgetfulness is not necessary. 

However, a person who sits after the third rak'ah and stands up for the fourth rak'ah needs to sit at once, salute after at-tahiyyat and perform prostration of forgetfulness. However, if he has prostrated after the fourth rak'ah, he needs to add one more rak'ah, making it a five-rak'ah prayer; he needsperform prostration of forgetfulness at the end of the prayer. Thus, his prayer becomes complete. Fard is regarded to have been completed since he has performed the last sitting.

The same thing is valid for those who stand up for the fifth rak'ah at the end of fourth rak'ah by performing or without performing the last sitting.

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