Qubes OS does not handle memory like other Linux Distros. It uses a complex system to manage memory. You could read more about it here.
That 4GB you see is allocated only to dom0 (used usually for management, storage and most probably for Audio and GUI/Video). Since you have only 8GB of Memory, there will be only 4GB left for other service VMs and AppVMs. You should manage your memory more carefully since you have limited memory.
- You could reduce dom0 memory usage (maybe to 2GB). Here is a forum thread on how to do it.
- You could reduce initial memory and maximum memory for Service VMs (sys-net, sys-firewall, sys-usb). I am personally using around half a Gigabyte for each of them.
- You could disable sys-whonix autostart and launch it only when you need it.
- You could install minimal templates and use them for Service VMs. Here is the official guide.
Whatever you do, always take notes of previous values. So you could revert them back to original value if needed.