As far as more RAM helping you, that depends on what you use the PC for. If you are a massive multitasker with 30 tabs open in your browser plus several other programs running then 4gb might not be enough. If gaming then 4gb is not enough.
Now, the other thing though is that the factory probably only put 1 stick of ram in the computer. Ram is ment to run in pairs so 1x4gb is actually 10-15% slower then 2x2gb or 2x4gb or 1x2gb + 1x4gb. Thus having 1 ram slot empty reduces performance.
With all that said, the absolute best upgrade you can do for your laptop is to replace the hard drive with an SSD drive. You will have a much faster system, faster boot times, and even gain a little extra battery life.