Prussia’s first stamps date from 1850. They were engraved by Eduard Eichens. Six years later, a new value was added to the set: the 4pf yellow-green. Although the portrait of the previous values was used, Eichens was no longer around to engrave the frame with the new value denomination, and it was H G Schilling who engraved the frame for this particular stamp. Although Schilling tried to replicate the existing frame as closely as possible, he used the word pfenninge for the value, rather than the word pfennige which had been used for the 6pf value of 1850. It is sometimes classed as an error, which is not really true, because both forms were accepted at the time.
You will find H G Schilling's database HERE.