The M4 Mac Mini is an amazing computer that will meet your needs well. The two questions that you will want to ask yourself are: how much RAM do I need, and how much SSD storage do I need?
For basic business tasks you may be fine with 16 GB of RAM but may want to consider 24 GB if you do things like keep dozens of applications, documents, or browser tabs opened at the same time or if you use photo or video editing applications frequently.
As far as SSD storage, you will need to evaluate how much total storage that you currently use on your Mac and any external drives and decide on how much additional space that you may need for the future. Remember that you should always have plenty of unused space on an SSD so that it can work efficiently. So, I would recommend taking the amount of storage that you currently use and configure your new Mac to have at least 50 to 100% more for growth. External drives are great for adding massive amounts of storage if you need that in your business, but for most users that don't have that need, you will find it much more simple and reliable to have all of the storage you need in the internal SSD. Only you can determine how much that may be though based on your current usage.
Finally, understand that your RAM and internal SSD cannot be upgraded later so choose your new configuration wisely.