You mean here:
There is no incrementing being done by those statements. You add 4 to i_1 and then throw it on the floor. And do the same with i_2.
Perhaps you meant to do
i_1 += 4;
i_2 += 4;
THOSE statements will indeed increment each of those variables by 4.
But this code makes no sense either:
(1) The syntax is illegal. You are using square brackets in an inaprorpriate place.
(2) You are comparing something (not sure what, because the square brackets are illegal) to a *STRING* '4'. Are you aware that the string '1000000' is *LESS THANT the string '4'? If you want to compare NUMERIC values, use numbers, not strings.
Finally, I don't know when or where this code fragment is supposed to run, but if it runs at any point in time *AFTER* the HTML page is finished loading, it will WIPE OUT ALL CONTENTS in the page, including even the code you show here. You can't use document.write
after a page is loaded.