What should a latecomer do to complete the 'Isha prayer if he enters the mosque and the Imam is in the fourth rak'ah, and what should he recite in the fourth rak'ah and the two rak'ahs he missed?

The preponderant view is that the rak'ah you catch with the Imam is the first rak'ah of your prayer, so you recite Al-Fatiha and a surah in it. What you complete after the Imam's taslim is the end of your prayer, so you recite Al-Fatiha and a surah in the first rak'ah after the Imam's taslim (which is the second for you), and in the remaining two rak'ahs, you recite only Al-Fatiha (because they are the third and fourth for you).