Chapter 3 of The Communist Manifesto by Marx and Engels tries to define communism not thorough it capitalist opposition (chapter 1 and mainly chapter 2 did that) but rather as distinguished from the closer socialist movement. The basic point is that socialism, unlike communism, is essentially counter-revolutionary since they just want progressive change that will only serve to perpetuate capitalism and its exploitative practices. What Marx and Engels are after in their Communist Manifesto and not a bandage but rather a deep and total change of the very structure of society the mode of production. The point and goal for communism is the root, the core, and that means to turn the table completely as bring down the whole system instead of just trying to improve on it. This sets communism apart from other socialist movements that might be preaching a seemingly similar gospel and demanding similar demands. Chapter 3 of the Manifesto also claims that while socialism in fact serves the interests of the bourgeois, it is communism that is really tuned in to the needs of the actual working class and that it is the instrument to bring about the end of the conflict between bourgeois and proletariat (see chapter 2 of the Manifesto)
Chapter 4 is the last short chapter closing the Communist Manifesto. Titled "Position of the Communists in Relation to the Various Opposition Parties" it discusses the relationship between the communist party and other parties and movements and Europe of the time. The rule of thumb is that the party supports anyone who wants to overthrow the existing ruling order, preferably by force.
The Communist Manifesto ends with the famous call: "Workers of the world - unite!"