Book 4a of the Key Words with Peter and Jane reading scheme presents Peter and Jane engaged in various activities and introduces 42 new words, such as 'then', 'house', 'make', and 'read'. After finishing this book, the child proceeds to book 4b. The scheme relies on gradually introducing and repeating key words to help children recognize them on sight, using the 'look and say' approach. Examples of key words include 'the', 'one', 'two', and 'he'.