#!/bin/bash -x
x=1
y=1
#change this for jpg or other formats. $1 is the folder where the images are. given as an argument.
for fname in $1/*.png
do
#for more than 999 files change to %04d
if [ $y -le 4 ]; then
	rm $fname
	y=$(($y+1))
else
	mv $fname $1/`printf "%04d.png" $x`
	y=1
fi
x=$(($x+1))
done