Between ages two and four, a whole lot of development and refining of skills occurs. At 2, toddlers can walk well, run with wild abandon while crashing into things, step up onto stairs or low platforms, and squat down to pick things up. At 4 they can jump from place to place, run with finesse and avoid obstacles while kicking a ball along with them, throw and catch things with (relative) finesse, and display solid balance; for example when walking along a low wall or something similar.
Intellectual development really accelerates, too. They will go from naming a few bodyparts, answering simple "what" questions, pointing at named items, and using a few words in a sentence, to: using toys and other people for abstract imagination play, answer and ask complex "why" questions, and having a vocabulary of hundreds and thousands of words.
Reading with your child in this period is excellent for their mental development, at 2 they'll enjoy hearing your voice and watching your facial expressions. At 4 they could even be reading along, or abstracting the characters in the story while you read. All reading will help improve their language skills, cognition, and vocabulary.