The printer’s manuscript has build rather than the 1830 edition’s fill. The 1908 RLDS edition restored build, the reading in 𝓟, to the RLDS text. The 1920 LDS edition restored build to the LDS text, either by conjecture or by reference to the RLDS text.
Internal evidence argues that the original text (and probably 𝓞) read build. The 1830 fill is most likely due to the typesetter misreading build as fill (since the visual contour for both words is similar). The following yea-clause uses the verb build (“yea even that great city Zarahemla did they cause to be built again”). This supports the use of build earlier on in the verse since yea- clauses typically restate or qualify a preceding statement.
Elsewhere in the text, cities are built (19 more times), as in these five examples referring to the rebuilding of cities (just as in 4 Nephi 1:7–8):
In one place (quoting from Isaiah 14:21 in the King James Bible), “the face of the world” can be filled with cities, but cities themselves are not said to be filled:
In other words, there are no examples referring to the filling up of cities.
Summary: The reading of the printer’s manuscript in 4 Nephi 1:7 (“they did build cities again”) makes very good sense and is consistent with other passages in the Book of Mormon that refer to building cities; the 1830 reading, “they did fill cities again”, is probably a visual misreading of the original manuscript’s build as fill.