When you "combine like terms", you are putting together terms that look similar. For example, 4n and n look similar, because they both have an "n". Imagine the n's are apples, and numbers without 'n' are oranges.
If you have 4 apples, and add 1 apple, you have 5 apples, or 5n.
If you have 6 oranges, and take away 2, you have 4 oranges. But, you can't put apples and oranges together, they are 2 different fruits.
So, you're left with 5n+4!