MySQL fulltext search with MATCH...AGAINST

I've built a fulltext index, ft_min_word_len is set to 4,  my search returns no results for "christmas" but several results for "Christmas". I couldn't find any documentation on this. Could someone tell me what's going wrong here? I've probably left off a setting variable...

My SQL procedure is:
BEGIN
  IF inAllWords = "on" THEN
    PREPARE statement FROM
      "SELECT   pk_product, t_name,
                IF(LENGTH(t_description) <= ?,
                   t_description,
                   CONCAT(LEFT(t_description, ?),
                          '...')) AS description,
                n_price, n_discounted_price, t_thumbnail
       FROM     product
       WHERE    MATCH (t_name, t_description)
                AGAINST (? IN BOOLEAN MODE)
       ORDER BY MATCH (t_name, t_description)
                AGAINST (? IN BOOLEAN MODE) DESC
       LIMIT    ?, ?";
  ELSE
    PREPARE statement FROM
      "SELECT   pk_product, t_name,
                IF(LENGTH(t_description) <= ?,
                   t_description,
                   CONCAT(LEFT(t_description, ?),
                          '...')) AS description,
                n_price, n_discounted_price, t_thumbnail
       FROM     product
       WHERE    MATCH (t_name, t_description) AGAINST (?)
       ORDER BY MATCH (t_name, t_description) AGAINST (?) DESC
       LIMIT    ?, ?";
  END IF;

  SET @p1 = inShortProductDescriptionLength;
  SET @p2 = inSearchString;
  SET @p3 = inStartItem;
  SET @p4 = inProductsPerPage;

  EXECUTE statement USING @p1, @p1, @p2, @p2, @p3, @p4;
END

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