Perhaps you don't quite understand what a RAM disk is. It's not a magical solution to boost computer's performance or increase its RAM. It allows you to have a super-fast disk in unused RAM, so generally slow disk operations e.g. on temporary files could be done in RAM and completed faster.
In your case, although you have 4 GB, it's likely that 2 GB is used by the OS and apps with only 2 GB left. So naturally only that unused space can be allocated to a RAM Disk. If you do so and allocate most free space to a RAM Disk, your system will have almost no free RAM left, which will instead slow everything down.
Perhaps
this article can shed more light on what a RAM Disk is and how it can be used.