file_1,2,3,4 refer to channel 1,2,3,4 ,maybe change the “file_1” to “CHA” is more clear .
the 4K storage is available for each channel means the storge for analog and digital channel that is :
for analog channel: when CHA and CHB are all working:4K8(bit)for CHA and CHB ;
when CHA or CHB is hiden: 8K8(bit)for the un-hiden channel;
digital channel: 4k*1(bit) for CHC and CHD.