inxi shows that only 3 GB are available as the TOTAL, although it finds
the 4 GB to be physically installed:
$ sudo inxi -m -x
Memory: RAM: total: 2.88 GiB used: 2.11 GiB (73.2%)
Array-1: capacity: 4 GiB slots: 2 EC: None max module size: 2 GiB note: est.
Device-1: M1 size: 2 GiB speed: 667 MT/s type: DDR2
Device-2: M2 size: 2 GiB speed: 667 MT/s type: DDR2
It could be a different problem (Windows may calculate RAM differently in different places, where did you check it?).
A similar tool for Windows is "Rammap".
With 32bit system you would never be able to use 4GB (because part of address space is used for MMIO by PCI-(express) devices).
With 64bit address space is much larger, but some ram is used by kernel. Some tools exclude such ram.