四合字母（tetragraph、或称四重音字、四重字），在字母系统上是由4个字母组合成的语音，而组成的语音与个别字母的发音不同。且能由四合字母表记出新的语音(音位)。

A tetragraph (from the Greek:τετρα-, tetra-, "four" and γράφω, gráphō, "write") is a sequence of four letters used to represent a single sound (phoneme), or a combination of sounds, that do not necessarily correspond to the individual values of the letters. In German, for example, the tetragraph tsch represents the sound of the English digraph ch. English does not have tetragraphs in native words (the closest is perhaps the sequence -ough in words like through), but chth is a true tetragraph when found initially in words of Greek origin such as chthonian.