FCP is only using 409 MBs of memory out of your 4 GBs. But whatever else you have open is using up virtually all the physical RAM you have. This is forcing the memory manager to page memory out to the disk-based VM file. That's a major reason for major slowdowns.
You have 4 GBs of RAM. How many applications are you running concurrently? If you look at Page outs: you will note the positive number in parentheses. That means your computer is hitting the hard drive a lot because the memory demands are too high. If possible cut down on how many concurrent applications you run or put more RAM into the computer, if that's possible.
About OS X Memory Management and Usage
Understanding top output in the Terminal
The amount of available RAM for applications is the sum of Free RAM and Inactive RAM. This will change as applications are opened and closed or change from active to inactive status. The Swap figure represents an estimate of the total amount of swap space required for VM if used, but does not necessarily indicate the actual size of the existing swap file. If you are really in need of more RAM that would be indicated by how frequently the system uses VM. If you open the Terminal and run the top command at the prompt you will find information reported on Pageins () and Pageouts (). Pageouts () is the important figure. If the value in the parentheses is 0 (zero) then OS X is not making instantaneous use of VM which means you have adequate physical RAM for the system with the applications you have loaded. If the figure in parentheses is running positive and your hard drive is constantly being used (thrashing) then you need more physical RAM.
Adding RAM only makes it possible to run more programs concurrently. It doesn't speed up the computer nor make games run faster. What it can do is prevent the system from having to use disk-based VM when it runs out of RAM because you are trying to run too many applications concurrently or using applications that are extremely RAM dependent. It will improve the performance of applications that run mostly in RAM or when loading programs.
Tell us about your project and its content. 4G is pretty marginal.