15 and 19 age difference

The other day my friend and I were having a casual conversation . Her and her boyfriend have been dating for a while and she bought up the fact that he was 19 when they started dating and she was only 15. I looked surprised and she asked me what was wrong and I told her the age difference was kind of creepy . Four years isn’t that long when you’re legal but she had just gotten out of middle school and he already been out of highschool for a year. She got offended and told me that wasn’t a bad age difference . Idk what do you guys think? Is that acceptable ?