There are a few things you can consider when purchasing a laptop under 400 dollars. One is the processor. A processor is important for a laptop because it will help you run your applications more smoothly. Another important factor is the size of the laptop. Laptops that are smaller or have a smaller screen size will be more difficult to use and will not be as powerful. Another important factor is the battery life. Laptops that have smaller batteries will be less powerful and will need to be recharged more often.