Four letters - one sound!

Solution 1:

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.
  The GH in through is silent. The single vowel /u/ in /θru/ is represented by the OU.

And there isn't a technical term for it, either, though you can make one up if you like.