I decided to go with this:
https://europeanautosource.com/antig...f8x-m3-m4.html
I spoke to EAS - they said someone there has an M4 and used this battery. Note that it actually includes the IBS which is cool (I had previously bought one so if I don't need that, I'll have an extra if anyone wants to get from me).
I ordered and it comes in next week. I won't be able to put in the car until that weekend so here's to "fingers crossed"!. I am really hopeful this works. total with shipping was $1034. Thats much cheaper than the BMW stock battery (who knows what brand) and even Braille.
Just need to register with Carly - hopefully that works too.