Vista with SP1 will SHOW that you have 4GB but will NOT be able to use all 4 GB of it in 32 Bit versions. That was a addition to SP1 where it would show that you had 4GB installed.
To know for sure. Start>Run>winver or Right click on Computer>Properties.
That will give you the definite answer if you are running 32 Bit or not.
It is impossible for 32 Bit to use 4GB. While it can report it it can not use it. That 512MB of space is reserved for IRQ's of the devices. To get the full effect of the 4GB of RAM you would have to format and install 64 Bit OS.