if you want fast speed, go for class 6 instead of class 4. label it class 4 certainly means the card can't attain 6MB/s speed for the class 6 rating. however certain high speed class 6 cards can have much faster I/O speed, since there isn't any high rating then class 6 on the SDHC rating. my SDHC microSD has a minimum 10MB/s transfer speed, on average it's 15MB/s (only dip to 10MB/s when switching chips for a moment).
another thing to consider is, cheap flash cards certainly means it's MLC flash, which is slower then SLC. SLC will give you a lot of speed and erase cycle, but cost is really high(3-4 times of MLC), you'd only see SLC in performance cards or Industrial grade cards.