You can't. PS can't do what the operating system doesn't allow and well, on a 64bit system, 4GB is simply measly...
What Mylenium says. These days 8 GB are considered the bare minimum amount to run Photoshop, and 16 GB are much better.
The Adobe published minimum requirements for Photoshop are laughably optimistic.
I'm not an expert, but this doc confirms, that your OP Windows7 Ultimate x64
supports 4MB, in fact up to 192 MB:
Best regards --Gernot Hoffmann
supports 4GB, in fact up to 192 GB:
Well, do you know what could be causing the problem?
If I'm not wrong, PS should see all my 4GB of RAM or at least 3072MB.
My OS does.