If I join a prayer in congregation with the imam at the 4th rakaah (I'm unaware) and he goes into his 2nd tashahhud (but the first for me new joiner) should I recite the 1st or the 2nd? Should I try to guess which one it is for him?

Ustadh AbdulAziz al-Haqqan

There is no need to guess. That sitting is only you following the imam, so you may recite the tashahhud, all of it and make dua as well if the Imam takes long to say the salam.