The reason you are not getting access to the full 4 Gb is possibly a limitation within your specific operating system and without heavy diagnostics I would not be able to tell you.
Based on Windows 10 documentation a 64-bit product can access anywhere between 128 Gb and 6 Tb depending on the version of windows you are using. Most Windows 10 systems our users are running are either Home (128 Gb) or Pro (2 Tb).
Scott