You can't read "a specific item" from a text file, because it doesn't have any implicit order - in fact even lines are just an interpretation of characters within the file: each time a newline character (or character pair, depending on the operating system) is encountered, that is the end of the previous line.
Your code assumes that the data is in lines, with data separated by commas (this is called CSV or "Commas separated value" data) and the only way to get "the fourth item from each row" is literally to read each line - which you do - and use Split - which you do - to break it into items. Once split, you can access each rows items via an array index into strArr.
Personally? I'd use a CSVReader: A Fast CSV Reader
] which will process teh data better than a "brute force" technicque like you use (as it supports text strings which may contain commas).