Anecdata: I’m a guy and a III. My son almost was a IV but we chose another first name and used my (and my dad’s and my grandfather’s) name for his middle name. My great grandfather had a different name, but I think his dad and maybe grandfather had the same name as me.
Not to offend, but I think it has declined because this naming of Jr, III, and so on has come to be associated with poor or rural people. For what it’s worth, I have a relative who is a IV
I’d agree the trend is waning if that’s what you’re getting at but I think it is baked into the history of this dataset and is also still significantly more common for boys than girls
Also, IV is really quite rare for boys too. Junior still super common.