How does the worshiper complete his four-rak'ah prayer if he joins the Imam in the last rak'ah, and is he required to perform the Tashahhud in every rak'ah?

The preponderant view among scholars is that what you catch with the Imam should be considered the beginning of your prayer. Thus, after the Imam finishes the prayer, you should perform one rak'ah in which you recite Al-Fatiha and a surah, then sit for the first tashahhud. After that, you perform two rak'ahs in which you recite only Al-Fatiha, without any sitting between them.