These four verbs can describe positive changes, but they draw attention to different things. The best choice depends on what changes and how.
- Improve is the broad, everyday choice for making something better or becoming better. It works with skills, health, services, and many other things. Daily practice improved her pronunciation. It can also stand without an object: Her pronunciation improved.
- Enhance usually emphasizes increasing a desirable quality, such as beauty, enjoyment, value, or effectiveness. Soft lighting enhances the room’s atmosphere. The room does not have to be bad beforehand; the lighting adds to its appeal.
- Ameliorate is formal and usually refers to making a bad situation or problem less severe. The new policy aims to ameliorate overcrowding. It is more likely in academic or policy writing than in casual conversation. It does not necessarily mean solving the problem completely.
- Augment means increasing an amount, size, or capacity by adding something. She took weekend work to augment her income. It is also relatively formal. The focus is on addition, not necessarily on better quality.
Some contexts allow more than one choice. Training can improve performance or enhance existing skills, with a slightly different emphasis. However, adding more staff may augment a team without automatically improving its work. These are typical differences in focus, not strict boundaries between unrelated meanings.