Over 30 UK species, very difficult to differentiate. 

Tetrastichus: Submarginal vein in forewing with one dorsal seta, propodeum with a carina (ridge) branched like an inverted "Y".
Aprostocetus: Submarginal vein in forewing with 2-5 (usually 3 or more) dorsal setae, propodeum with an unbranched medial carina (ridge).
Quadrastichus and Oomyzus also share the single seta on the submarginal vein with Tetrastichus, but they lack the Y-shaped propodeal carina. 

Unless identified by a recognized expert, explain how the specimen was identified and include photos of key features. 

Widespread.

Unknown.

Hansson, C., & Schmidt, S. (2020). A revision of European species of the genus Tetrastichus Haliday (Hymenoptera: Eulophidae) using integrative taxonomy. Biodiversity data journal, 8, e59177.