when written after the variable, the "++" operator fires AFTER the other stuff on the line is done. so we go around the loop a few times. eventually, we enter the loop with i equalling 3. the first thing that happens is we to the test. since args.length() is 4, we pass.
AFTER WE DO THAT TEST, we increment i to 4. you now try and get the element at index 4, which throws the exception.
You should always try running code like this when what the author says doesn't make sense. if there is an exception thrown, take out the line and try and figure out why. you could take out the "args[i]" and just print i. you'd see it printing values 1,2,3 and 4. then you can try and figure out why it's that instead of 0,1,2,3