If a song is in the key of Bb and I wanted to play a G instead of the Bb, what fret do I place the capo?

That is what I thought but I watched a YouTube instructional video and they said the song was in the key of Bb yet they placed the capo on the 4th fret and played a G.

The people providing the video are active musicians I believe.

The notes go A Bb B C Db D Eb E F Gb G Ab (A)
Making the capo go up one fret higher makes the key one note higher.

Think about it.
But boys will be boys and girls have those eyes
that'll cut you to ribbons, sometimes
and all you can do is just wait by the moon
and bleed if it's what she says you ought to do
Exactly, so if the song you're playing is in the key of Bb and you wanted it to feel as if it were in G, placing a capo on the 3rd fret makes the Bb feel like a G, F would feel like a D, and so forth. The only way a capo on the 4th fret would result in a Bb feeling like a G would be if the guitar was tuned down a half step.