Ubuntu 32bit only supports 3 GB of RAM so if you have 4, only 3 will be used. You can use the 64bit version for all 4GB of RAM memory to work, or follow the instructions in this post to make the 32bit version of Ubuntu to work with 4GB or RAM.
Firstly, please note that to achieve this, you must install the Ubuntu Server Kernel which has Phisical Extencion Address Enabled (PAE) enabled thus allowing Ubuntu to see all 4 GB of RAM. You can also compile the 32 Kernel desktop version yourself, and enable PAE but that's difficult to explain so I'll not cover that :). Once, that was easily done with Kernelcheck, but now kernel.org changed something which made Kernelcheck to stop working. Another solution would be to just install the 64bit Ubuntu.
Installing the Ubuntu Server Kernel is basically for lazy people and should only take 1-2 minutes. If you wish to continue with this solution, follow the steps below:
Open a terminal and paste the following command:
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http://webupd8.blogspot.com/2009/09/4-gb-of-ram-in-ubuntu-32bit-how-to.html