It has nothing to do with bookmarks importing or exporting.
It's about Smart Bookmarks. That 4 is simply the version of Smart Bookmarks, nothing more. You won't see 1, 2 or 3 unless you have an older version of Firefox.
What are Smart Bookmarks? Recently Bookmarked, Recent Tags and Most Visited are the default ones.
Setting the value to -1 disables Smart Bookmarks, setting the value to zero restores the default set.
More information about Smart BookmarksCyberNotes: Create Your Own Smart Bookmarks in Firefox 3
I know is says Fx 3, it is still applicable. That is how long Smart Bookmarks have been around.--
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