How much RAM do I need to run a 4GB VM on Windows or macOS?

16GB is the minimum. With 8GB total, the host OS uses 2–4GB, plus browser and apps, leaving only about 4GB for the host while the VM takes another 4GB—both fight over RAM, causing swapping and lag. On Windows or macOS, 16GB lets the host use 8–10GB comfortably, gives the VM its 4GB, and keeps everything snappy without swapping.