A U1 or U3 class of SD card will be good for 4K quality video as they both have good enough write speeds; however for a dash cam that constantly fills then re-writes over the data, you will need an 'endurance' or 'industrial' card.
These are designed specifically to be in 24hour constants use/re-writes.
Endurance cards also withstand 'random' write slowdown.
For example, copying a single file to an U1 or U3 class of card will write at about 100mbits/second.
But when you mix it up and have random write (where the card is written on constantly in all different areas of the storage) this can drop to 3 or 4mbits/second in some cases.
Endurance cards are specifically designed for this and only reduce to 40-50mbits/second.
An average 4K video is 28mbits/second.
So an Endurance class SD is what you want to look for in a dash cam - as they're designed to be in constant use and still maintain very high write speed when in random write.
The difference between U1 and U3 classes are mainly the speed.
I'm not aware of any U3 endurance class micro SD cards.
A recommended card for high quality resolution or dual recording camera is something like this;
Samsung 128GB Pro Endurance U1 Class