According to SteamSpy, the title has over 153,000 owners. As SteamSpy numbers are generally agreed to be accurate, you can infer that about a quarter of its audience bought the game on PlayStation 4.
As of this writing it's the number-three seller on Steam's "Top Sellers" chart, and it's the number-one "trending" game on SteamSpy (which is based on a variety of data including newness, sales, and other factors undisclosed by site creator Sergey Galyonkin.)
It's far from the only recent indie hit, as pointed out on Twitter by Cosmic Star Heroine developer Robert Boyd:
SteamSpy indie hit update: Hyper Light Drifter (88k), Enter the Gungeon (131k), Stardew Valley (915k), Undertale (1.4 mil).— Robert Boyd (@werezompire) April 11, 2016
There's also a brief but interesting blog on these numbers (and what they mean -- regarding the so-called "indiepocalypse" -- from veteran independent dev Rob Fearon.