4 chips for 16MB would actually double the bandwidth if it was banked surely?I would also imagine two half density chips would maintain performance by having twice the bus width. Willing to be proved wrong, though!
So I doubt it is for speed reasons. I suspect that using a half density chip rather than doubling the number of chips is for keeping cost down.
That said just saw this... Someone upgraded their 8GB to 16GB... and upgraded the flash as well.
source: https://appleinsider.com/articles/2...-ram-ssd-on-your-m1-mac-mini-but-you-shouldnt