Where does the theme of love and loyalty appear in act 4?

The theme of love and loyalty is presented in act 4, scene 2 during Brutus and Cassius's argument before the Battle of Philippi. When Cassius arrives at Brutus's tent, the two generals begin to argue over the fact that Brutus punished one of Cassius's men for taking bribes. Brutus then challenges Cassius by saying that it is a shame that he would become corrupt because that is the same reason they murdered Caesar. Brutus then laments at the fact that Cassius denies giving him money, and Cassius responds by saying the messenger was late. Brutus's anger stems from the fact that he feels Cassius is not loyal. Cassius then accuses Brutus of not loving him anymore. Cassius is deeply hurt by Brutus's accusations and tells him,

Come, Antony, and young Octavius, come, Revenge yourselves alone on Cassius, For Cassius is aweary of the world—Hated by one he loves; braved by his brother; Checked like a bondman, all his faults observed, Set in a notebook, learned, and conned by rote To cast into my teeth. Oh, I could weep My spirit from mine eyes (Shakespeare, 4.2.96-104).

After Brutus witnesses Cassius's display of love, he tells his friend to put down the dagger. The two men proceed to shake hands and vow to remain loyal allies for the rest of the civil war as they fight against Octavius and Antony's forces.