Marx says that history is the history of class struggles. What are four examples that he gives of "class struggles"?

To answer this question, take a look at chapter one of The Communist Manifesto. In the opening lines, Marx and Engels write that all history is the history of class struggles. They then provide the following four examples:

1. The struggle between Freeman and Slave.2. The struggle between Patrician and Plebeian.3. The struggle between Lord and Serf.4. The struggle between Guild-Master and Journeyman.

Each of these examples dates from a different historical period. The Patrician and Plebeian, for example, come from Ancient Rome, while the Lord and Serf belong to the Middle Ages. By using these examples, Marx and Engels show that every period of history, from the earliest recorded civilizations, has been one of continued class struggle. Specifically, there is a conflict between the rich (who own the means of production) and the poor (who are either enslaved or must sell their labor).
For Marx and Engels, this explains why Communism ought to be adopted across Europe. Without Communism (and a classless society), they argue that the future is certain to be one of continued class struggle in which the proletariat face further exploitation.

At the beginning of Section I of the Manifesto of the Communist Party, Marx briefly lists a number of historical class conflicts, including "freeman vs. slave" and "guild-master vs. journeyman." Marx takes care to emphasize that ancient societies included a lot of different and subtle gradations of class. For example, Roman society contained "patricians, knights, plebeians, slaves." On the other hand, modern society has much larger, more simplified classes.
Marx spends most of his time focused on these modern class conflicts. The first is the bourgeoisie vs. the nobility. For Marx, the nobility were essentially feudal. Their enemies, the modern bourgeoisie, represented the evolution and consolidation of many of the other subordinate classes from the peak of the age of feudalism (like "vassals, guild-masters, journeymen, apprentices, serfs"). Before describing the conflict between these classes, Marx fleshes out the evolution of the bourgeoisie, showing how this class grew in power and influence.
The bourgeoisie grew more influential as a class because they were willing to embrace more modern ways of conducting economic business. This allowed them to gain an advantage over the nobility, whose value system was not entirely compatible with the emerging forces of capitalism. Marx summarizes it thusly:

The bourgeoisie, wherever it has got the upper hand, has put an end to all feudal, patriarchal, idyllic relations. It has pitilessly torn asunder the motley feudal ties that bound man to his “natural superiors”, and has left remaining no other nexus between man and man than naked self-interest, than callous “cash payment”. It has drowned the most heavenly ecstasies of religious fervour, of chivalrous enthusiasm, of philistine sentimentalism, in the icy water of egotistical calculation.

Marx is grateful to the bourgeoisie for—as he sees it—sending the nobility to the trash heap of European history. However, because Marx sees history as dialectical, as the bourgeoisie destroyed the nobility, so must it also be destroyed in order for the world to attain a truly classless society. This is where the central class conflict of the Manifesto comes in.
That central conflict is the bourgeoisie vs. the proletariat. The proletariat are laborers who must sell themselves to those who own the means of production for a wage. This process of selling the self for a wage makes the proletarian alienated. Marx writes:

Owing to the extensive use of machinery, and to the division of labour, the work of the proletarians has lost all individual character, and, consequently, all charm for the workman. He becomes an appendage of the machine, and it is only the most simple, most monotonous, and most easily acquired knack, that is required of him.

Throughout the Manifesto, Marx urges the proletariat—which is growing in number and has the advantage of being concentrated in large groups in cities—to overlook the things that divide it. Marx believes the proletariat must overlook things like religious or national differences and collectively realize that the true enemy is the bourgeoisie and the bourgeois conception of private property. As he says at the end of the Manifesto, "The proletarians have nothing to lose but their chains. They have a world to win. Working Men of All Countries, Unite!"
https://www.marxists.org/archive/marx/works/1848/communist-manifesto/ch01.htm

In the first chapter of The Communist Manifesto, Marx gives seven examples of class struggle: slaves versus freemen, patricians versus plebeians, feudal lords versus serfs, guild masters versus journeymen, the bourgeoisie versus the nobility, the lower middle classes versus the bourgeoisie, and proletarians versus the bourgeoisie.
He oversimplifies the history of social interactions by stressing social conflicts while bypassing cooperation among various social groups and by reducing all social distinctions to distinctions of class. This analysis reflects the unstable political and social environment of nineteenth-century Europe, where industrialization and the growth of a market society precipitated the breakdown of traditional social ties and cultural values; after these broke down, only material interests remained. In The Communist Manifesto, Marx calls these interests the “cash nexus.”
Marx portrays this dissolution of social interdependence as a destructive but ultimately progressive phenomenon. In his opinion, by destroying old certainties, capitalism emancipates individuals from patriarchal subjugation and traditional loyalties. Thus, in his analysis, capitalism revolutionizes society and prepares it for the establishment of communism.