SQL only takes memory as it needs it. Right now the server only needs 3.8 GB of memory as it needs more it will take more. If you need it to take at least 7GB every time it starts up and "never" releases it then you can set you "min server memory" property to 7 GB. Have you setup your "max server memory" too. I would recommend you give the OS at least 2GB and start from there.

I would recommend you download this ebook (or buy the hard copy if you want). Read the whole book, but you can skip ahead to the memory section to better explain what is going on and what options to set. Be careful with "lock pages in memory" if you don't understand what is going on behind the scenes.