There is also the mount settings. Mostly, USB/SD devices are mounted with the flush option. As a result, the buffered data is flushed to device more frequently, resulting in the "stuttering" behavior you are seeing. I experience this myself when copying big data sets to flash devices. Personally, I don't think there is much you can do about it. Flash discs on the other hand, have controllers that help level the I/O, especially when writing to disc/device.

Finally, I have several 32GB class 10 SD cards, and when I start to copy a large data set to them, they run incredibly fast! After awhile, they start to exhibit the behavior you are seeing, leveling out at about 100mbps (mega bits, not bytes per second), 12.5MBps. If I copy a smaller (but still sizable) data set, that will fit into cache, the copy is immediate, but then I notice that the device is still busy as the system flushes the cache. I think the lesson here is "Don't believe everything you see!"... or think you see. :-)

There are a lot of factors at play here - the amount of RAM in your system that can be used as cache, the type of device (class 2...10), the interface (USB 3.0 in this case, assuming the system also has a USB 3.0 port), how new the device is, ... Try writing not 4GB, but 10+ GB to the device, and then run the sync command and see how long it takes before it finishes. THAT is the salient factor here. Even in your example, write 4GB, and then run sync and see how long it takes to finish flushing the data to the device so that it is removable.