Windows 7 Starter doesn't support more than 2GB of memory. The only way you can use all 4GB is to upgrade to a different version of Windows. Even then, Windows will reserve some for system functions, so you'll see something like 4.00GB (3.75GB usable).
This is directly from Microsoft:
Physical Memory Limits: Windows 7
The following table specifies the limits on physical memory for Windows 7.
Windows 7 Ultimate
4 GB - x86
192 GB - x64
Windows 7 Enterprise
4 GB - x86
192 GB - x64
Windows 7 Professional
4 GB - x86
192 GB - x64
Windows 7 Home Premium
4 GB - x86
16 GB - x64
Windows 7 Home Basic
4 GB - x86
8 GB - x64
Windows 7 Starter
2 GB - x86
N/A - x64