What is meant by 4 –12% or 8% SDS-PAGE?
Do the percentage values refer to the percentage of acrylamide in the gel?

Yes. The 8% gel is 8 g acrylamide per 100 mL. The “4-12%” gel is a gradient gel, which are useful for separating proteins over a large range of sizes. Read more about the uses and formulations of gradient gels here.