I have a box with 4 12inch subs on top. Im going to be putting a slot port in the center of the box wondering how to tune it

find out how many cf your box is (HxWxD/1728=cf)
deside what tuneing you want
and find out how much port area you need and then type that all into here
http://www.carstereo.com/help/Articles.cfm?id=31

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Thanks big help for sure