Digital Workshop

hi all
I have a file called data.txt
the file has many lines.
I can add new line to the file by using
txtObj.GotoNextLine()
txtObj.WriteLine("hello world!")

this is work fine. but what I want is:
suppose that I have 5 lines in the txt file

line1
line2
line3
line4
line5

NOW I want to add "hello world" after the line 4 and before the line 5
this is the whole story.
I appreciate any help.

There is probably a much more elegant way than this idea, but one way is to read from the file, one line at a time, 1,2,3,4 and write line by line to a new file(data.txt... in a different directory), then add the new line, 5, then the other lines 5,6,7, from data.txt, finally overwrite the old file with the new one......or something like that, very clunky.