One wonders here if the final relative clause originally read “which was spoken of by the mouth of the angel”. In Mosiah 2:32, we have an example where Oliver Cowdery initially omitted the preposition of in a similar relative clause:
This example clearly shows that Oliver sometimes omitted the of from the phrase “to be spoken of by X” (see Mosiah 2:32 for discussion). As further support for emending Mosiah 4:11, one could refer to one more example of this phraseology in king Benjamin’s discourse:
If Mosiah 4:11 is to be emended by inserting of, there remains the question of whether the of should come between spoken and by or at the beginning of the relative clause (as in “of which was spoken by the mouth of the angel”). Although the use of of at the beginning of the relative clause seems awkward, there is evidence for the initial placement of of in relative clauses involving passive forms of the verb speak followed by an agentive prepositional phrase:
Note especially the second example; there Oliver Cowdery, when he first took down Joseph Smith’s dictation for this passage, omitted the of that occurred at the beginning of the relative clause. Also note that the agent phrases in both these examples specifically refer to that which has been spoken “by the mouths of the prophets”, which is similar to the language in Mosiah 4:11 (“by the mouth of the angel”); that is, all three involve the noun mouth. These two examples from Alma 40 thus support the placement of the of at the beginning of the relative clause in Mosiah 4:11.
Despite these arguments for emending the final relative clause in Mosiah 4:11, there is evidence that the earliest reading may be correct if we interpret the text as stating that the mouth of the angel spoke something rather than of something. The verb speak does not require the preposition of, as in the following passages:
Notice how both of these passages refer to something being spoken by the mouth of someone, just as in Mosiah 4:11. And in all three cases, there is no of for the verb speak in the earliest textual sources. Thus the original text in Mosiah 4:11 is perfectly interpretable if we consider the text as referring to the mouth of the angel as having spoken “that which is to come”. The critical text will therefore accept the earliest reading for Mosiah 4:11 and not emend the text by adding an of in the relative clause.
Summary: Maintain in Mosiah 4:11 the current reading “that which is to come which was spoken by the mouth of the angel”; the final relative clause does not need an of for the verb speak.