I mean, yes, it is a red flag, but if that's the only thing so far, for me, it doesn't have to mean the end of the relationship.
I've not had anyone say it after 4 days, but when I've had partners who've said it sooner than I think is reasonable (like, less than 3 months absolute minimum), I have literally said to them "No, you don't, you couldn't possibly, you don't know me well enough yet. But I'm enjoying getting to know you and maybe it'll develop in time."
And then, for me, it's settled down and some of those have gone on to be lovely, multi-year long term relationships.
But it does make me look harder for other signs, and if someone was showing any other signs of love bombing or rushing or controlling, then yes, I would be very wary.