02-23-2020 08:00 PM
The SanDisk SDXC cards rated at U3 Class 10 are ample for this camcorder. I'm using both the older 95 MB/s cards as well as newer 170 MB/s ones. Note that those are the typical read speeds; write speeds are a bit slower. However, these ratings are in mega _bytes_ per second and not megabits. Note that the bit-rate for 4K in the G50 is 150 Mb/s. So divide by 8 to get MB/s which is around 19 MB/s (far below the card's write speed; even at sustained writes).
Be sure to fully format the cards in-between major recording sessions. Cards that become fragmented can slow down.
When you mention that the 4K video is very stuttery, what was the frame rate? Also, what stabilization mode were you using? Note that if you have stabilization mode on while also using a tripod, that can lead to issues.
When playing the video back, what is the refresh rate of your display? I record in 23.976p and use displays with proper pulldown. Sometimes if using the wrong settings in post, you can end up with jitter as well.