What should someone do who forgot one rak'ah of a four-rak'ah prayer after the Tasleem, and had joined the prayer late by two rak'ahs?

If someone gives the salaam after three rak'ahs in a four-rak'ah prayer, and then remembers before a long interval has passed, he should return to the prayer with a fresh intention and say the Takbir. Then he should stand up with the Takbir, perform the fourth rak'ah, sit for the Tashahhud, and perform two prostrations of forgetfulness (sujud al-sahw). However, if a long interval has passed between his salaam and his remembering, he must restart his prayer.