Running the big Lion on 4GB RAM is like using one-size-too-small shoes. They may fit, but won't be comfortable. Suggest an upgrade to 8GB (~$45) or bettery yet to 16GB (~$75). Corsair, Crucial, Kingston are all good brands, just make sure what you pick is Mac Certified.
Now back to the issue (which derives from above), try this: http://support.apple.com/kb/TS1473
OS X uses memory very efficiently. You paid for 4 GB and OS X will use all it wants to use. There is no benefit to having memory remain unused.
Decoding the system memory pie chart above: Green means it is unused. Blue means it has been recently used, and remains available to the same process that used it but since released it. If the process requires its recently released memory again, it remains available to load quickly. For purposes of memory that remains available to use for whatever needs it though, green = blue.
As you can see from the above this system has 16 GB memory so it has essentially unlimited resources available for most reasonable purposes, but it appears as though it is using 4 GB. It's really using only about three quarters of that, and remember OS X is using all it wants to use. This is approximately equal to what you described: it's normal.
Examine the values for page outs and page ins. The total number is not important; it is only the recent activity that matters. If page outs are constantly occurring that means your system is constantly writing memory to disk, which will degrade performance. This will be associated with nearly constant disk activity.
SSDs such as in the new iMacs and MBAs effectively hide this performance degradation.
As I wrote
For purposes of memory that remains available to use for whatever needs it though, green = blue.
More info in addition to your link (for the OP) Using Activity Monitor to read System Memory and determine how much RAM is being used