These are the statues on the Outdoor Art tab of the artworks category. If you have all of the DLC there are now more than four statues. The ones you need for the achievement should be the first four displayed.
The Water Bearer (free), The Conversations (free), Epic Progress (7,500), and Fortuna Imperatrix Mundi (15,000).
The first 3 take 2 tiles each and the last one takes 4 tiles - so you need at least 10 tiles of outdoor space available within your lot boundaries.