You will get sluggish performance as you only have 4 Gb RAM in your machine. I'm lucky to have 16 Gb RAM. I allocate 4 Gb RAM and 2 CPU cores for most VMs in Virt Manager unless I want to see how little lightweight OS's can cope with. I've just posted an answer to someone on how to import a .vhd into Virt Manager. Will edit this post with link when I find it.