What should parents expect from their baby's development and feeding between 4 months and 1 year?

Our baby is growing fast and we want to make sure we are supporting their development in the right way during this stage. What are the key developmental milestones to look out for between 4 months and 1 year, including movement, communication, and social behaviour? When and how should we introduce solid foods alongside breastfeeding or formula? What changes in behaviour and sleeping patterns are normal during this period and what signs should prompt us to seek advice from a health professional?