What are the four branches (ministries) of government and with what are they concerned?

First of all, there's the Ministry of Truth, where Winston Smith works. The name is something of a misnomer because the ministry isn't concerned with the truth at all; rather it is concerned with the dissemination of lies and propaganda. In this totalitarian society, the truth is simply what the Party says it is, even if it means that two plus two equals five. As part of its concerted assault on the truth, the Ministry also distorts, manipulates, and destroys historical documents as part of an attempt to rewrite history. It's not enough for the Party to control the present and the future; it must control the past as well.
The Ministry of Love is another grotesquely misnamed department of state. "Miniluv" is concerned with the torture of anyone daring enough to step out of line. The whole purpose of such torture is to inculcate political dissidents with love for Big Brother, as Winston Smith discovers for himself when he pays a visit to the terrifying Room 101.
In a society where "Freedom is Slavery," and "Ignorance is Strength," it's no surprise that the Ministry of Peace is actually devoted to prosecuting Oceania's increasingly bitter conflict with Eastasia.
Last but not least, we have the Ministry of Plenty. As you've probably already guessed by now, this is another department of state whose title is little more than a sick joke. For there's not much in the way of plenty in Oceania. The country is poor, with chronic shortages making the lives of ordinary people an absolute misery. Although "Miniplenty" is supposed to be devoted to the production of food and other essential goods, it actually spends most of its time churning out bogus statistics that purport to show how well the economy is doing and how, under the wise, benevolent rule of Big Brother, Oceania is a veritable paradise of abundance.