Often writing systems lack a letter for a sound and that lacuna is covered by a digraph, two letters representing one sound, like English SH and CH. German has a trigraph, SCH which stands for the same single sound that Croatian, Czech, and Slovene represent with the one letter Š.
Danish seems to take the n-graph cake in this respect wasting FOUR letters for the same consonant sound with a quadrigraph. We find it in a few surnames like Schjødt where the first four letters represent the single sound that SH represents in English and Š in those Slavic languages that use the Latin alphabet.
The interesting thing about this quadrigraph is that it contains the digraphs for the same sound in several Indo-European languages using the Latin alphabet: CH in French, SH in English, SCH in German (a trigrph), and SJ, which is used in most Danish words.
SCHJ is the only quadrigraph I know of. I know some consider combinations with the GH digraph in English to be quadrigraphs, e.g. eight and though. These are not quadrigraphs, though, for they represent two sounds. Eight represents the sound [eyt] and though, [ðow], i.e. the two sounds of diphthongs, a vowel plus a reduced consonant. This is not the case in Danish. It is true, as I say, that the Danish quadrigraph occurs only in surnames like Schjeldal, Schjelderup, Schjødt, Schjønberg, and Schjønning but they are true quadrigraphs.
Does anyone out there speak a language that has another?