On the SSDs... with Apple's embedded SSD controller, the only NAND that works is very limited and pretty much has to be sourced from iPhone or other Mac teardowns with the same NAND type. It's not standard unfortunately..... That being said...
One really nice thing with MacOS 15 and later is that now you can even install applications on an external drive. So you can get a super nice, fastest 40GB/s SSD solution like our Express 1M2 - up to 8TB... and only use the internal drive primarily for MacOS with documents, apps, etc - mapped to the external. This saves a lot vs. internal factory capacity + goes with you to next systems too.
The M4 is wicked fast... although an M1 Max is still a little over the M4. And and M2 or M3 vs. M4 - smaller and smaller real world to even notice unless you're doing intensive A/V work or, maybe, gaming.
Not sure what you're coming from going into an M1 Mac mini - but it's likely it's going to be super snappy and meet all you need/want + support should be on it for at least the next 4-6 years OS wise I'd expect. Apple does all this Apple Intelligence 'made for' marking on systems released after AI was added.... anything with M1 or later does all the Apple AI stuff..
So - new M4 is nice - but you may not even notice the difference and you've got that M1