Cambridge University Press has what seems to be a pretty decent translation, which on the whole agrees with yours. Reading the whole encyclopædic article entry (of “Chapter” 38) in which that #4 section is contained helped me to contextualise Isidore’s understanding of the different notions about time.
From what I’ve read elsewhere, there was originally no Latin word which expressed a concept like time which has no end
(or further yet time with no beginning
as well either), to the effect that this [“endless time,” i.e.] cannot be the original meaning of aeternitas
, “eternity.” Aetas
, the root word from which aeternitas
is derived, Isidore uses to mean “life” or “lifespan,” which appears to be exactly the original meaning of Greek aiōn
. Here’s Isidore’s expertise on the subject, after which he goes on to define and describe the “ages” of world history (in the subsequent 2 chapters). Your #4 section is highlighted below.
The Etymologies of Isidore of Seville
xxxviii. Periods and ages (De saeculis et aetatibus)
1. Saecula consist of generations, and hence the term saeculum, because they ‘follow’ (sequi) one after another, for when some pass away, others take their place. Some call a saeculum a period of fifty years, which the Hebrews call a jubilee.
2. It was for this reason that the Hebrew who – on account of his wife and children, and loving his master – was kept in slavery with his ear pierced, was commanded to serve for a saeculum, that is, up to the fiftieth year (see Exodus 21:5–6).
3. An ‘age’ commonly means either one year, as in the annals, or seven, as one of the ages of a human, or a hundred – or any period. Hence an age is also a time composed of many centuries. And an age (aetas) is so called as if it were aevitas, that is, something similar to an aeon (aevum).
4. For an aeon is a perpetual age, whose beginning or end is unknown. The Greeks call this an αἰών, and they sometimes use this word for ‘century,’ sometimes for ‘eternity’ – and from Greek it was borrowed by Latin speakers.
5. The term ‘age’ properly is used in two ways: either as an age of a human – as infancy, youth, old age – or as an age of the world, whose first age is from Adam to Noah; second from Noah to Abraham; third from Abraham to David; fourth from David to the exile of Judah to Babylon; fifth from then [the Babylonian captivity], to the advent of our Savior in the flesh; sixth, which is now under way, to when the world itself comes to an end. The succession of these ages through generations and reigns is thus reviewed.
, by Stephen A. Barney, W.J. Lewis, J.A. Beach & Oliver Berghof, with the collaboration of Muriel Hall