I am not good enough at maths/programming to implement the change.
Cut the crap. You don't have to be good at math to program. The Arduino is.
tried changing... to...
Look at the reference page. The for loop consists of three simple parts - the initial condition, the continue condition, and the after-each-iteration part.
You got the right idea about what you need to change in the after-each iteration part. Now, what is the correct initial condition to start at 3 instead of 0? What is the correct continue condition when i is greater than 0, instead of when i is less than 4?