Please correct me, if I'm not getting correct education.
Your teacher is right. If the 4rth thing is last, it is better to say last. But actually, to me it seems just as good to say fourth sometimes.
I am going to tell you five steps to writing a paragraph. First, make sure you think of one clear message you want the reader to remember. Second, write a sentence that expresses that idea. Third, you should add another sentence that gives an example or more explanation. Fourth, you should go back and precede those two sentences with a topic sentence. Fifth, go to the end of the paragraph and add a conclusion/transition sentence.
In this case, it is good to use fifth instead of last, but last is okay, too.
Notice that I do not use the ly.
That ly is terrible! Just write first, second, third, fourth,...
:-)