Sorry, but that sounds like nonsense. I have seen sd card speed benchmarks posted here from our phone showing significantly higher data rates than 4 mb/s. It will most likely not increase external transfer speeds though (usb mass storage). I know that the class of the card only refers to the write speed, but at least read throughput of 10+ mb are deffinitely possible. I unfortunately have a card that only reads AND writes at about 4 mb/s, and there is definitely room for improvement there!
Swyped on my Galaxy 3
Edit: show me this results.