The original text in this passage switches several times between the singular and plural. Part of this variation was removed by Joseph Smith in his editing for the 1837 edition. In the last part of the passage, he replaced the plural they and them with the singular he and him, in accord with the immediately preceding use of generic he (“and blessed is he that is found faithful unto my name at the last day”). Joseph probably made this change because the conjunction for connects the last part of this passage to that preceding sentence.
On the other hand, the use of the plural them at the end of verse 18 (“and signs shall follow them”) was not emended to him. One could argue that the them here is actually a mishearing for him, especially since them is pronounced /ßm/ in casual speech (for discussion of this possibility, see under 1 Nephi 10:18–19). Yet the phraseology here in Ether 4:18 paraphrases the language placed at the end of the Gospel of Mark (as well as the identical language in the last chapter of Mormon):
Thus the them near the end of verse 18 is very likely correct.
Similarly, one can argue for the original plural them later in Ether 4:19. First of all, we have a corresponding King James passage that occurs in the plural, albeit with the second person plural ye and you rather than with the third person plural they and them:
There is a similar passage earlier in the Book of Mormon text, and in that case we have third person plural pronouns:
Of course, the earliest textual sources support the use of the plural they and them in Ether 4:9, yet these parallel uses in Matthew 25 and 2 Nephi 9 provide further evidence that the plural forms are correct. Thus the critical text will restore the plural they and them in Ether 4:19.
Summary: Maintain in Ether 4:18 the plural them in “and signs shall follow them that believe in my name”, even though this clause is surrounded by the generic singular he; similarly, the original they and them in Ether 4:19 should be restored; in both verses, parallel language in the King James Bible and elsewhere in the Book of Mormon supports the plural usage.