What about development environments, they take a lot of ram if you have multiple windows open.
Video editing does take a lot of memory, but it's not the only thing that wants memory
Also, remember that to use 4Gb you'll need a 64bit OS as a 32bit OS will not be able to access all of it due to mapping of IO cards spaces into the upper regions of the 4Gb address space. 2.5-3Gb is all you can expect on the average 32bit system. Mine gets about 2.7Gb