With 4GBs of RAM, you can forget Ready Boost all together. The overhead you buy will be counterproductive. You have to understand that ready boost does not only write the pages to your stick, but also to your pagefile (for security reasons in case you pull the stick).
If you look at the bottom right graph in Resource Monitor > Memory tab, you will probably find that there is very little paging activity to begin with. So why buy the overhead.