I have a question. I have a 2009 GS450H with a blown sub. I got a replacement but it's for a 2013 GS. Can I use it in place of the actual sub? They look identical but the part number is 861600ww40 instead of 861600ww20.

 

You could have just repaired your subwoofer by replacing the worn lip. There is a member who made a video on how to do this, and the part is pretty cheap

 

Looking at the pictures online,
2009 subwoofer center is concave (surface curves inward)
and 2013 subwoofer center is convex (surface curves outward).

in 3rd gen GS, the subwoofer grill is built into the rear deck in the center,
and rear deck is flat, but looks like 4th gen's subwoofer, may protrude
above the rear deck.

Here is how I repaired the Mark Levinson subwoofer for $15.
In this video, you can also compare your 2013 subwoofer closely for compatibility chances.