Its likely that you cant your CPU cant keep up with encoding AIC that fast. Its a lot to ask.
Two things to try.
a) Run Black Magic Speed Test and see if you can write uncompressed 4K. Note the rough average of your disk in MB/s.
Uncompressed 4K runs roughly 620MB/s (3180×2160 @ 30Hz, RGB 4:4:4)
b) If your disk isn’t fast enough, try other codecs that are not as compressed, like Pro Res 4:4:4 or 4:2:2 HQ, or Animation (depending on your disk throughput).
This is ultimately about balancing how much disk throughput you have with how much encoding your CPU can handle in 1/30th of a second or so.