Some people may think that this type is the same as the previous type which is not true. The first type is that a trustworthy narrator narrates an addition and attributes it to the Prophet (peace be upon him) and make it part of his saying.
As for this type, a narrator mentions the Hadith and adds to it from his own wordings without differentiating between his words and the words of the Prophet (peace be upon him) and thus it remains unknown.
The scholars of Hadith name this type "Al Mudraj" which means that a narrator inserted his own wordings inside the words of the Prophet (peace be upon him) without differentiating between the two, and the people think that all words belong to the Prophet (peace be upon him).
For example, the Hadith of Ibn Mas`ud in which the Prophet (peace be upon him) took him by hand and taught him Tashah-hud, he said: "Say At-Tahiyyatu Lillahi... [Greetings are to Allah]" then mentioned At-Tashah-hud until the end then said: "When you say that, you have finished your Salah and if you want to leave, do it or you may sit if you want." So, his words: "When you say ..." until the end is the words of Ibn Mas`ud as an explanation from Ibn Mas`ud because the difference between the two phrases were mentioned in another narration where a narrator mentioned the Hadith until the end of Tashah-Hud then the narrator said: Ibn Mas`ud said: "When you say that..." So, the narrator differentiated between the words of the Prophet (peace be upon him) and the words of Ibn Mas`ud.
And the addition of a trustworthy narrator is accepted as was mentioned in the fourth type.