Well, Michael 4 and Victoria 4 are separate figures. But with that product you can make Genesis take on the appearance of Michael 4 or Victoria 4.
One difference is, some products that require Michael 4 or Victoria 4 won’t work on Genesis, even with the M4 or V4 shape. If you have questions about specific products it’s best to ask…but the basic rule is:
Skin textures for V4 and M4 can be used on Genesis
Clothing for V4 and M4 can be AutoFit to Genesis (results vary but are usually good, except for shoes and some long dresses and similar items)
Hair, poses, and props for V4 and M4 can be used on Genesis, but may require some manual tweaking to work properly
Morphs and expressions for V4 and M4 won’t work on Genesis, unless you own Genesis Generation X (GenX).