I am not sure what "across" refers to here. Does this sentence mean each person take four tests, or each person take one test and the total number of people of four tests is 163?
"Across" here means that the total number of people for the 4 tests was 163. "Across" is being used as a preposition/adverb here. Which means from one side to the other side of something. In this case from one test to the other, or you can thing if it as meaning "encompassing" or "including". Ex: Across all nations people used this brand of toothpaste. Ex: We want our motto to be understood across all languages.