The FIP (Federation Internationale de Philatelie) definition of Postal Stationery is:

"Postal Stationery comprises postal matter which either bears an officially authorised pre-printed stamp or device or inscription indicating that a specific face value of postage or related service has been prepaid"

 
4 øre wrapper. The imprinted stamp indicates that a specific value (4 øre) has been prepaid - and therefore qualifies as postal stationery.

If you want to read more about postal stationeries, I reccommend these articles (in Danish) from the Danish stamp magazine DFT:

Article 1: Postal Stationery: What is it - what is it not? (IN DANISH)
Article 2: The different types of Postal Stationery (IN DANISH)
Article 3: How do you collect Postal Stationery? (IN DANISH)

I reccommend the following in English: This article: "What do you know about postal stationery", from the website of United Postal Stationery Society and the following article on Linns.com:

Postal stationery - an introduction

The first postal stationery in Denmark is issued in 1865. The stamp imprints on the envelopes are not "bi-colored" and therefore not a part of this website.

2 skilling envelope from 1865

6 øre postal card to Brazil

8 øre letter card sent domestic 12.5.1892

2 skilling wrapper sent domestic 2.1.1873