One of the most difficult things to do is answering questions like this. The language is often confusing. My advice is to take things one part at a time and learn the keywords.
The first part of your question asks for an algebraic expression. This indicates that we are not writing an equation with an equals sign but just looking for an expression.
The next significant part say 400 less than.
This indicates that we will have some value - 400.
( If the question had just said 400 less ( something ) for example this would be 400 - something. )
The last part of the question says the product of 4 and a number n. You need to recognize that product is an indicator for multiplication, so that we are multiplying 4*n.
Now we can put each of the parts together to get the expression:
4n - 400
My suggestion for getting better at these types of problems is to review the keywords that indicate the type of operation and practice, practice, practice.
The webpages below both have pretty comprehensive lists of keywords, as well as sample problems and quiz type problems. This will allow you to practice and make sure you are getting the question right.