Advent comes from the Latin word adventus, which means “arrival” or “coming.”
It refers to the coming of Christ. There is no 4 in the name, no 24, 25 or whatever else.
Advent is a liturgical season, not a set of 4 Sundays. Neither 4 nor 25 are "in the name."
I thought it was meant to be like an Advent Calendar, which is normally 25.
Not in all traditions. E.g. in Germany Christmas Eve is typically the last day so you only get 24.
Yeah, in Sweden too.
Norway, too.
And even in the places where it’s 25 days, there’s plenty of advent calendars that only have 12 doors - though they’re typically budget versions of expensive calendars (eg a dram of whisky behind each door)
Falsehoods programmers believe about time #604: all advent calendars have 25 slots
Winter Solstice is usually (not always) on 21th December.