I've settled on the following word list to determine if 20 of 26 characters are present within 4 guesses:<p><pre><code> thank
fuels
crowd
gimpy
</code></pre>
If there are fewer than 5 letters present, "bevvy" will eliminate another two ('b' and 'v'), leaving j, q, x, and z as the remaining possible characters. These occur rarely (though "pizza", "bijou", and "vixen" can be challenging targets. More usually, it's words with either doubled characters, such as "onion" or "lalai", or those which have viable anagrams ("spams" and "spasm", "donor" and "rondo") that will throw me.