The Class number is actually megabytes per second of stream data (IOW, a continuous flow), so a Class 4 card can do 4mbps, a Class 10 card can do 10mbps. With UHS cards, Class 1 is 10mbps and Class 3 is 30mbps. For 4k video you need a UHS Class 3 card (that's what they're designed for). For standard HD video (1080i), a Class 4 card is a little faster than needed, so it'll do fine. (The speeds that the card is rated at are worst case minimum speeds, so a Class 4 card may actually be able to sustain 5mbps write speed - just don't count on more than the class rating.)
BTW, there are only 2 factories in the world making the chips for those cards, SanDisk and Samsung. Everyone else buys chips from them. I don't have any personal knowledge of whether they're buying top-of-the-line chips or rejects, but I've been in this business long enough to know who knows what he's talking about and who's passing on rumors, and I'll only buy SanDisk cards. Samsung cards are probably just as good, but if you ever have a problem, SanDisk is very easy to work with - just use their live chat on their website and they'll do whatever they can to fix a legitimate complaint. (I broke the covering on a microUSB card with my fingernail - I didn't complain to them because that's my fault, not theirs.)
Yes, it costs a little more (unless you catch a great sale - 64GB UHS-1 SanDisk cards were on sale about a week ago for about $28), but the customer service and peace of mind is worth it. And when I say "if you ever have a problem", I mean "if". I still have working 256MB cards and I think, somewhere, I have a 32MB stick - which is so old it was considered revolutionary at the time. And the last time I used it, it still worked. I have a few dead "other brand" cards in my "reminder" drawer - a reminder to never buy them again. You want the card to outlast its usefulness. The 16GB cards I used back in Gingerbread days are still working, but a Class 4 16GB card today? It wouldn't hold even 3 nandroid backups.