Four is the definitive "truthful word" because it has four letters. "Unfour" is also truthful as it doesn't have four letters, but is untruthful as it's not a real word. Following an excursion into the un-section of the dictionary I found heaps of truthful and untruthful words, as well as some both truthful and untruthful--like unfour but real words. Here's a sampling, mostly of linguistic interest.
Word Ways: Vol. 39
, Article 23.
Available at: http://digitalcommons.butler.edu/wordways/vol39/iss1/23