Insert guitar nerd moment. The don't even look at it part. VERY rare guitar. This is a very old video that talks a bit about it.
To nerd out a bit more, a Fender Bass IV is a six string bass. Most bass guitars are four strings. This one is tuned "generally" to "guitar" tuning not bass, and gives you some amazing tones....sounds. A baritone is not as low, or deep as a bass IV. Different strings across all of them.
I have a bass VI and the thing is just amazing. If you want that specific tone (there is that stupid word again) nothing else is going to come close.
Try to suffer through the video, you will hear some of the tones this thing can put out.
I've never touched one, or looked at one in real life. Its got a unique sound.
Used to keep a strat set up with heavy strings that could be tuned down to where the low e was an a, and was still playable. Similar concept just not to that extreme.
It was kinda in the middle of a guitar and bass, like Luke warm water...