In my x64 laptop I have 16G of RAM. I see arcGIS is using 4G, I have 11G of RAM available. Also, I was trying to process an 8G raster and I've got a not enough memory error.
Is there anyway I can make more RAM allocate/available for arcGIS usage?
Although your Windows installation is 64-bit, ArcGIS Desktop is still 32-bit software and can't use more than 4GB of RAM in a single process. However, you can install the ArcGIS 64-bit Background Geoprocessing addon, that will let you run most geoprocessing tools in 64-bit mode and they can use more than 4GB RAM in that case. There are some exceptions, but it doesn't look like they'll be a problem in your case:
Tools that do not run in the background include the following:
Tools inside the Metadata conversion toolset
Tools inside the Geodatabase administration toolset
All Coverage tools Tools which create packages
Graphing tools (64-bit geoprocessing only; these tools work in traditional 32-bit background processing)
Custom script, model, or function tools where the author has disabled background processing
Just install the 64-bit geoprocessing addon for your version of ArcGIS (you need to be running v10.1 or later) and enable background geoprocessing under the Geoprocessing > Geoprocessing Options menu.