I am looking in Sidi manual (sorry can not picture it, too blury) and only two screws i found are related with Dura Ace Shimano cleat for millennium 2 sole.
It says: Place the DA Shimano cleat fixing it with original SIDI srews A5x12mm, use the SIDI srews provided with the spare part purchased separately.
See picture
That 4th hole should be for Look Keo memory cleat (srews M3x5mm)

i guess, as i can see from instruction sheet. Do not ask what is memory cleat, but i see it goes under Look Keo cleat and should be fix with 3x5mm.
See picture, memory cleat goes under midle 4th hole
You are having millennium 3 sole?
I just found UK Sidi site, it is all there, Italian site is awful, but nice looking.
http://www.sidiuk.co.uk/Technology/CleatAssemblyMillennium2Sole.htm
Hope this help, i guess.