"Our results demonstrate effects of letter and quadrigram frequency above and beyond those of bigram frequency, suggesting that all of these levels (Dehaene et al., 2005), not just bigrams (Grainger & Whitney, 2004; Whitney, 2001), may be useful subcomponents of visual word recognition."
However the claim that their data provides evidence for quadrigram detectors is tenuous, at best. The claim comes from the comparison of frequent bigrams forming rare quadrigrams vs. frequent bigrams forming frequent quadrigrams. However, this comparison is confounded with the pronounceability of the stimuli. The rare quadrigram stimuli were not pronounceable, whereas the frequent quadrigram stimuli were pronounceable. Thus frequent-quadrigram stimuli were much more likely to yield partial activation of lexical representations, and therefore any difference between the two may reflect different levels of lexical activation, rather than quadrigram activation. This is supported by their finding that only words and frequent quadrigrams yielded significant activation of posterior middle temporal gyrus, a region associated with lexico-semantic processing.
On the other hand, the contrast between frequent letters forming rare bigrams vs frequent letters forming frequent bigrams provides does not suffer this confound, as both types of strings were not pronounceable. Differences between between these types of stimuli were found in middle/anterior left fusiform. Binder et al. (2006, Neuroimage) also found sensitivity to bigram frequency in this area in another fMRI study. These results support the claim of multi-letter units, such as open-bigrams, and are difficult to explain under models that do not include them, such as Davis's SOLAR model and Gomez et al.'s Overlap model.