I did more than 4 😛
I started dating an ex when I was 17.8 (nearing birthday) and she was 22.8 (nearing birthday as well), so a 5 year span. It worked fine for us as I was more mature than my age (in relationships) and she was slightly less mature in relationships and life. Neither of our families were really keen on it in the beginning though, it took a solid few months to prove to them all that we meant it and it wasn't just a joke or for screwing. When they finally bought it 🙂D) we were fine, mostly. My mother never really cared for her which can make things hard, but her and I were fine.
It is about maturity, but it's also about societal norms as sad as that may be. 17 is only 1 year past 16 which just sounds far too young to even talk about for me, and I'm only 21 too! Not sure what it is about that 1 year, but it certainly matters.
I prefer older women so dropping to 17 would be freaky-weird, but I think it's doable if you both really care for each other. Be prepared to defend the relationship at times though, sad but it will need to be done.