Why does the M20 piston have four ring grooves but only takes three rings? My current piston has three rings, when I ordered new rings, my order only came with three and a NOS piston that I have only has three rings. Is this correct?

Andy read this thread.
http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1151713&cmd=show

Thanks Matt, didn't think to do a search.