When praying Salat, before the Tasleem greetings "Assalamu alaikum wa rahmatullah السلام عليكم ورحمة الله" and turning your face to the right and left, what is the fourth Tashahhud to be recited following At-Tahiyyatu التحيات, Allahumma salli 'ala اللهم صلّ على, Allahumma barik'ala اللهم بارك على?

I recite "Allahumma inna nastainuka, wanastagfirukka اللهم إنا نستعينك ونستغفرك..." Is this valid?