Brad, I can't speak to Ordinary Farm, not having read it, but I can wholeheartedly recommend Tad Williams' "Tailchaser's Song" as a "read-to-young-kids" book. Many years ago I read it aloud to my three kids (then around 5, 7 and 8 years old), one chapter each night, at bedtime. They loved it, and it remains talked about in our family to this day (they are now 26, 28 and 29 years old). There were only two sections of the book which were (slightly) problematic for them:
1) The Prologue - It is written in the style of an ancient myth (of the cat's society), and so was difficult for the kids to appreciate or understand. But it is brief.
2) A scary section - there are a few chapters towards the end where the book's main characters enter into a hellish domain of the book's villain (sort of a feline Mordor). I remember my kids worrying about the heroes' safety (although some of their worry was probably played up in order to convince their father to read "just one more chapter tonight, PLEASE!!"
With these two (very) mild caveats, I would heartily recommend "Tailchaser's Song" as something to be read to a 4-year-old, particularly if the 4-year-old loves cats.
I will also add that the hours I spent reading to my kids are among my most precious memories. Good for you for reading to yours!