var currentImg = 1;
var Id = 'img01';

function prevnext(dir) {
	document.getElementById(Id).style.display = "none";
	if (dir == 'prev') {
		currentImg--;
		if (currentImg < 1) {
			currentImg = 4;
		}
	}
	else {
		currentImg++;
		if (currentImg > 4) {
			currentImg = 1;
		}
	}
	Id = 'img0' + currentImg;
	document.getElementById(Id).style.display = "block";
}