well, i haven't had stuttering with 4k video... which encodes at a bitrate of around 40Mbps, which is around 5MBps (Mbps and MBps are different for those wondering). So 10MBps you're getting "should" be fine for 4k.
Out of curiosity, what format is your sd card in? Fat32, NTFS or ExFat? Some people have had issues with ExFat (i haven't at all). Did you format the car before popping it in?
Also, just because the chipset supports it, doesn't mean the device itself will. If 4k video is 5-6MBps, why would the phone need to go any faster? Granted, I have more speedtest experience using sd/microsd cards in DLSR's but the same applies. We also have no clue is some sort of caching is taking place or not. To top it off, any apps installed onto the sd card that can run in the background or any apps running in the background? Using the task switcher doesn't give you all the apps that are possibly open. after closing all apps, tap the back arrow a few times.? I know a couple of people who noticed better 4K recording once they made sure no apps were running.
The samsung card I use (UHS1, 40MBps read) has had no issue with 4K recording.