Dear ,
I have the same model city 15,ivtec manual (20 k driven), changing oil after 4 months never void warranty
But if your usage is less miles / month you can consider changing after 6 months (Maximum)
my recommendation would be 4500 km or 6 months which ever comes first change oil.
Honda claims that their Oil filter can go until two oils e.g 10,000 km for filter change.
I strongly recommends changing oil filter with every oil change
Thanks