I found a rectangular white stone in southern lake Huron with 4 brown horizontal lines ground into it. I was told it was a native American artifact. Do you know what this is and the meaning behind it?

Oh, how interesting.
 
If you could get more information from the person who told you it was a Native American artifact you could start an internet search to start narrowing things down, as there are a few tribes that surround the Great Lakes. You might also try to take a picture of it and do a reverse google image search. It is a long shot, but you might be able to find something out about the item.
 
From there you could start contacting local museums or local and university libraries to see if they could provide you with more information.
 
After a quick internet search, I found this Milwaukee Public Museum page that you might find interesting.
 
Have fun on your adventure of discovery!
 
Take care,
 
Auntie Manda