Slightly overlaps for reasons of coherence, apologies …
 … And the imitation by prose alone or by metred words (verse); and by metered words either
Thus if they were to produce some medical or naturalist text in meter, they would be customarily likely to name it accordingly [as poetry]; but Homer and Empedocles have nothing in common besides the meter, wherefore to name the poetic work justly, the latter [Empedocles] would be a naturalistic text rather than a poetic work. Similarly if anyone should craft an imitation joining quite every one of the meters together (just as Chairemon crafted the mixed rhapsody “Centuar” from quite every one of the meters),
Concerning these things, then, let this distinguish the matter.
There are some
 I agreed with Christ’s punctuation more than Fyfe (the Loeb edition of 1965 that I am using) in that the thought (with Burglar’s translation): “all make the imitation in rhythm and speech and harmony” is the beginning of a somewhat disorganized digression on three different ways that “rhythm, word and harmony” can be seperately or jointly employed in different poetic “imitations”: instrument-playing, dancing and word-compositions.
 Phrase literally “bare words” often means “prose”, which makes the most sense here.
 Editors have inserted “anwnumos” meaning “nameless” here based on an Arabic text so that the translation would be something like, “a single kind of those metres employed up to the present day which happen to be nameless …” This makes sense with the explanation Aristotle then embarks upon, but I think obscures the real point: the next section is an explanation for why he had to be so vague as to use the phrase “a single kind of those metres which happen to be employed at the present time”—he is being intentionally vague because the terms currently in use (Athens, circa 4th c. BC) are inadequate, which he is trying to correct. Understand the next section as Aristotle’s deconstruction of current categories of labelling according to appearance instead of (what he wants to do) labelling according to the means of “crafting an imitation”.
 EDITED. Re: Burglar's efforts to bring my overtranslating back to earth, I have dumped "poetically create an imitation" and now use will use:
"craft" as a translation for the verb "poiew" (as in 'craft a poem')
"poetic work" as a translation for the noun "poeisis"
"craft an imitation" as a translation for the repeated phrase "poeiw mimesin".
This is bound to change, so stay tuned.
 I use “naturalist” in the classical Aristotelean sense of “natural philosopher” or “natural scientist” which I think is still more accurate than saying simply “scientist.”
 EDITED. This "in which" is a translation of the Greek preposition "en" which is usually just translated as I did here with "in". In the koine Greek of the New Testament, a very frequent 'agreed upon by the authorities' translation of this preposition is "by" as in "by means of". That is more the sense that I get from this sentence as well as the earlier statement it is referring to (see the end of I.4--Burglar's translation: "all make the imitation IN rhythm and speech and harmony"). "In" works perfectly well because we also say for instance a poem "in meter", but the important point for Ar. seems to be that they are imitations IN verse the same way a painting would be an imitation IN oils or IN watercolors -- these different categories of POETIC imitation are separated by the medium they rely on for that imitation.
So, a brief summary of how I understand Aristotle so far: poetic imitations should be so called no matter what form of rhythm or harmony or word, whether of prose, verse or combinations thereof they use, because what matters is whether or not they are “poetically creating an imitation” through a certain set of means. Categories for distinguishing these poetic imitations from each other:
1. the kind of imitation (ie., rhythms, words, harmonies)
2. the object of imitation (heroes or philosophers or lovers, etc)
3. the manner or style of imitation (ie., within the “kinds”, ‘words’ could use prose, verse, or combinations thereof)
Distinguishing poetic imitation from other imitative crafts, such as painting, whereby one also imitates (see I.4a), is the means of imitation (thus the insane frequency of the dative case in these pages). Painters imitate by means of copying postures of figures and colors; the poetic imitations by means of everything you can do with “sound” (I.4b).