I wanted one for a thing some months ago, and the 4s were out of stock most places and silly prices where they were available. 3s were in a similar state. I had a 3 spare but wanted the extra spec (USB3 and a bit more CPU umpf). I ended up getting a 400 instead as they were readily available at the time. When the 4s become sensibly available I'll swap it out for one and the 400 will become a toy for other purposes.

EDIT: sweet Jesus. Just checked the current state on Amazon and the 400s are listed at £117 for just the base unit, more than the £91 I paid for the full kit (the only in-stock option at the time) six months ago, the 4Gb 4 is £173 (next day prime) or £155 (up to two week delivery), and the 8Gb is down for a whopping £290! (next day, no cheaper slower options). The only Pi3 showing as in-stock ATM is £100. Maybe I should hunt out my spare 3 and make some drinking money from it!

I've not checked other sources.