Chapter 2.3 was primarily about cases. Panini provides the rules that govern which cases are tied to the karakas. The world of cases is practically unlimited since they aspire to represent the whole world. The rules aim to connect the semantics of the verbs and words with the roots and suffixes of the language. This is why context (arthha) carries importance in the Chapter 2 rules.
Chapter 2.4 is concerned with number of dvandva. We learn many rules that drive whether dvandva can be treated as singular or not. A remarkable rule is that of small organisms - the collection of which should be considered singular. The rules get into details of what is small and what exceptions might the rule entail.
2.4.18 onwards are rules that decide which tatpurushas would be treated as neuter gender. These are often driven by meaning/sense (2.4.21,2.4.24). The gender is considered 2.4.31 onwards. After a brief commentary on ArdhadhAtuka , the chapter talks about lakars (laT, lan, lin) transforming into luk or otherwise (2.4.40 etc.) - often in the context of certain suffixes.