Note that if your attacker knows the dictionary used, the effective strength of a passphrase isn't based on its string length but on the number of individual words within the phrase.
Six words gives an effective key strength of 2^77 bits.
I'm assuming they stuck with four to make them memorable by actually humans. Even then I have my moments where I type in an adjective form instead of a noun for a particular word/variant.
Coming up with grammatical combinations (adjective noun verb adverb preposition article noun, say: "brown dog ran quickly past the barn") might create more memorisable patterns, though at a cost of key strength.
Taking this to its logical conclusion, we'd have poetic stanzas of epic sagas randomly generated for mnemonic power utilising rhyme and meter....