In this passage an extraordinary error was introduced into the 1849 LDS edition: namely, an intrusive that between now and when. What is especially surprising is that this totally unacceptable reading persisted in all subsequent LDS editions until the 1920 edition. There are no other examples of “now that when” anywhere in the history of the text. Here in Alma 18:4, it is possible that the following “now I know that” led the 1849 typesetter to accidentally insert the extra that after the now earlier in the passage.
Summary: Accept the current (and original) reading in Alma 18:4 without a that between now and when.