Three consonant tetragraphs exist in English that are more commonly sounded as two separate digraphs. However, when used in word-initial position they become one single sound:
⟨chth⟩ is pronounced /θ/ as in chthonian. Pronounced as two digraphs /kθ/ in autochthonous.
⟨phth⟩ is pronounced /θ/ as in phthisis. Pronounced as two digraphs /fθ/ (or /pθ/ by some) in diphthong.
⟨shch⟩ is pronounced /ʃ/ as in shcherbakovite, a mineral named after Russian geochemist and mineralogist, Dmitri Ivanovich Shcherbakov. It is used as the transcription of the Cyrillic letter Щ and usually read as two separate digraphs, /ʃ.t͡ʃ/ as in pushchairs or /s.t͡ʃ/ as in Pechishche, a place name in Belarus.
⟨illi⟩ is used to write the sound [j] in a few words such as médaillier[medaje].
In addition, trigraphs are sometimes followed by silent letters, and these sequences may be confused with tetragraphs:
⟨cque⟩ is found for [k] in words such as "grecque" and "Mecque", where the trigraph cqu is followed by the feminine suffix e.
⟨eaux⟩ is found for [o] when the silent plural suffix x is added to the trigraph eau.
⟨tsch⟩ represents [t͡ʃ], which is a relatively uncommon phoneme in German but appears in some very common words like deutsch ("German"), Deutschland ("Germany"), Tschechien ("Czech Republic"), and tschüss ("bye").
There are several sequences of four letters in the Romanized Popular Alphabet that transcribe what may be single consonants, depending on the analysis. However, their pronunciations are predictable from their components. All begin with the ⟨n⟩ of prenasalization, and end with the ⟨h⟩ of aspiration. Between these is a digraph, one of ⟨dl⟩ /tˡ/, ⟨pl⟩ /pˡ/, ⟨ts⟩ /ʈ͡ʂ/, or ⟨tx⟩ /t͡s/, which may itself be predictable.
⟨s-ch⟩ is used in the Puter orthographic variety of the Romansh language (spoken in the Upper Engadin area in Switzerland) for the sequence /ʃtɕ/ (while the similar trigraph ⟨sch⟩ denotes the sounds /ʃ/ and /ʒ/). It is not part of the orthography of Rumantsch Grischun, but is used in place names like S-chanf and in the Puter orthography used locally in schools again since 2011.
⟨thsh⟩ is used in Xhosa to write the sound [tʃʰ]. It is often replaced with the ambiguous trigraph ⟨tsh⟩.