Brute forcing the "correct horse battery staple" space
The reason for the above is that, although the English language is vast, and it would be difficult to brute force a random 4-word brain wallet, most people are not going to select words for a brain wallet at random.
Most people will select words that are easy to remember because they coincide with a meaningful event, or are otherwise meaningful to the person. This is not random, and as such can be easily be "guessed" by hackers.
If you were to create a brain wallet of 4 words randomly selected from 171k English words, it would be one possibility out of ~855 million trillion possible combinations. However, if the brain wallet is created from some set of words in a book or bible verse, the possible combinations is reduced by multiple magnitudes.