They differ in the number and specifications of ports. And they can be placed in different ways for Mac Mini M4
Model: MINI4PRO

USB A 2.0 x3
HDMI port
SD/TF card slot
3.5mm headphone jack
SSD slot
Put Mac Mini horizontally
Model: MINI218BPRO

USB A 2.0 x2
USB A 3.0 x1
USB C 3.0 x1
SD/TF card slot
3.5mm headphone jack
SSD slot
Put Mac Mini vertically