Re-did my experiment, how to properly mention it?
This is something to discuss with your advisor, but I can make a suggestion.
Some research that is based on constructed models turns out to be questionable if the model lacks stability and resiliency (robustness). There is a branch of science/math called catastrophe theory in which small changes to a system can produce dramatic, non-continuous, results. One way to guarantee that this is not the case is to tweak the model in small ways and determine if the outputs are also similarly small. If not, the model is too sensitive and may not give usable results.
So, what you have done, can be considered a validity check on your model and can be presented as such in a separate section. You have gained some evidence that the model is stable in some dimension. This could come after section 4 and before section 7, depending on how you want to write it up.
But your advisor's advice would be valuable.