The German Empire (1871-1949) had a decimal currency, with denominations that we consider "normal". Sure, there was the 3 Mark coin, but that was due to the fact that the value was equal to the pre-Empire taler. Other than that, nothing odd. And then, in 1932 ... a 4 pfennig coin is introduced. Huh?
In the late 1920s/early 30s many countries went through economically rough times. (Black Friday, recession, etc.) The German chancellor Heinrich Brüning decided, in late 1931, that a new coin could motivate people to purchase more consciously and control their expenses. Dealers and manufacturers would lower their prices, and all this would help the economy.
Many decisions of Brüning's government were not made by parliament approved laws but by so-called Notverordnungen - emergency decrees. That also applies to the 4 Reichspfennig coin; it was created by a Notverordnung dated 8 Dec 1931. A little later the piece was issued:

(Image from Wikipedia)
The 4 pfennig denomination had been in use in various countries that later became the German Empire, in the 18th and 19th centuries. (Prussia even had one until 1872, I think.) But those were parts of different mostly non-decimal setups. Now, in the 1930s, the denomination was considered strange by most. There were lots of ironical comments, articles and political cartoons about the "Brüning Taler" or "Poor Heinrich" coin. Lots of other nicknames too.
People did not like the piece, banks did not want them, also because this copper coin was fairly large and heavy - 5 g, 24 mm. There were a few attempts at enforcing its use; wages were supposed to be partly paid with (fifty!) 4 Rpf coins for example. Ultimately the coin was not successful, and we would nowadays think of it as a naive economic approach anyway.
The nazi government then did away with the coin; it ceased to be legal tender on 1 Oct 1933. Until 1935 or so it could be returned and exchanged. Quite a few of those 50 million 4 Rpf coins survived however, and they are not really expensive, usually in the low two-digit euro range ...
Christian