Yes, you can safely use the 4-pole audio jack extender with a normal pair of 3-pole headphones.
If you are curious about more technical details, please read on:
The audio jack of a traditional stereo headphone has only three poles (or conductors), referred to as Tip, Ring, and Sleeve (TRS). They direct the signal to the left stereo channel, right stereo channel, and ground, respectively.
To make headsets with built-in microphones backward compatible with 3-pole audio jacks, phone manufactures split the sleeve into two conductors, and refer to the four conductors as Tip, Ring 1, Ring 2, and Sleeve (TRRS), respectively.
The former two still direct the signal to the left and right stereo channels. For ring 2 and sleeve, there are two different standards:
CTIA / AHJ, which maps ring 2 and sleeve to ground and microphone, respectively. This includes Apple devices, most Android devices (including HTC, latest Samsung), most Nokia, Microsoft (including Surface, Lumia, and Xbox One controller with chat adapter), Sony (Dualshock 4), etc.
- OMPT, which maps ring 2 to sleeve to microphone and ground, respectively. This includes old Nokia (including Lumia series), old Samsung (2012 Chromebooks), old Sony Ericsson (2010 and 2011 Xperias) and Sony (PlayStation Vita).
In either case, if you connect a 3-pole headphone into a 4-pole jack or a 4-pole headset into a 3-pole jack, the microphone input is shorted with ground, so it will work properly without distortion.