Have you tried following acreditations for your KS4 EAL students?

Let me know, how they went!

http://www.cambridgeesol.org/exams/general-english/ket.html
http://www.cambridgeesol.org/exams/general-english/pet.html
http://www.cambridgeesol.org/exams/general-english/fce.html
http://www.cambridgestudents.org.uk/subjectpages/english/igcsesl/

Thanks