No, you need to check the Activity Monitor. Not "About this Mac": that just tells you how much is installed, the AM informs you how that installed memory is used.
My computer has same issue (unibody late 08 2.4 ghz). I putt now 8GB and shows 7.75 - i take out and put my old 2x2GB and show 3.75. This showed only in activity monitor; in abouth this mac its normal.
The good news that it works very good with 8 GB RAM