I realize that the principle behind "phonics" is that English spelling does represent something
about pronunciation; unfortunately, what it actually represents is Middle English pronunciation.
Thus, this isn't a situation where "four letters make one sound", because
- letters don't make sounds, at least not in Modern English.
English spelling simply doesn't represent English speech;
- and even if letters did make sounds, the
GH at the end wouldn't make any sound at all.
GH in through is silent. The single vowel /u/ in /θru/ is represented by the
And there isn't a technical term for it, either, though you can make one up if you like.