Have you tried changing the yolo specific parameters in the script. I think the problem you are experiencing is related to incorrect anchor use. It seems to me that the preset parameters were optimally set for yolov3-tiny and therefore work great when testing on yolov3, despite it being a lighter and less complex model (24 layers as opposed to 38 on v4). I got pretty much the same result as you on v4.
As for the disconnecting, ensure you are running the latest version of depthai. @erik might know more 🙂
Hope this helps,