FFmpeg mosaic with hardware acceleration on Quadro P4000
Regarding,

Cannot find a matching stream for unlabeled input pad 0 on filter Parsed_hwupload_cuda_17


in

[tmp3][lowerright] overlay=shortest=1:x=720:y=480;hwupload_cuda


you have to replace the ; with a , else the overlay filterchain is terminated and the hwupload starts a new chain.

Input options like -hwaccel cuvid -c:v h264_cuvid only apply to the input immediately following them. These options have to be prefixed before each input for which they're intended.

scale_npp is a CUDA scaler, so for a hardware-decoded stream, no filter is required to transfer frames. After scaling, frames will have to be transported to system memory using hwdownload,format=nv12.