A customer sent me an STL file that has 4 pieces laid out on the print bed, but I can’t print all 4 at once. How (what software) do you recommend I split these pieces up into individual STL files so I can print them 1 at a time?
Meshmixer - free (after you sign up for an Autodesk account) and easy to use. Plane cut will be what you need to use, make sure you keep both sides of the split and then export the individual parts out.
You can also use the Edit, Separate Shells menu option.
It is usually used for parts that are made from multiple shells but does not care if the parts are already phycially separated. it still sees them as shells in one part.
If you have Simplify3D you could also use it. From the menu Mesh -> Separate connected surfaces.