Littleleaf Lilac (Syringa microphylla) is a natural dwarf version of Lilac that only gets 4-5 feet tall.
These are 3rd year seedlings, 6 to 12 inches tall, bare root.
Photo credit: 阿橋 HQ, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons