Please take a look at this and see if I am understanding you. Am I on the right track?
ls -l /your/dir | awk '{print substr($NF, index($NF,"_")+1, 4)}' | sort +0n | awk 'BEGIN {getline TMP1; while(getline TMP2 > 0) if(TMP2 > TMP1 + 1) for(i=TMP1+1; i < TMP2; i++) print i; TMP1 = TMP2}}'