CFrame Part -4 Y Of my character Torso CFrame?

Ok Lets say i made a part Instance.new("Part" , Workspace). Now i want it to go under my feet EXACTLY in the middle (I'm using 6Rig Character). So i would use the Torso's CFrame in a loop

Ok but The thing is i want it exacly under my character feet (so i can make like a platform to walk on or whatever) So i remmber i can do somthing like -4 Or somthing after the CFrame or position im not really sure but it will take the Torso CFrame and do (-) from the Y for Exacly under my feet and in the middle (Hope i didnt made it too complex lol)

while true do wait()
	Workspace.Part.CFrame = Workspace.amit123456.Torso.CFrame
end