Sorry Bookie - goint to have to disagree with you there:
"G-AB report passing 4000 ft in the climb"
"[snap, crackle, pop] Passing 4000 ft in the climb, G-AB"
"Ah, roger G-AB, break, G-CD climb to altitude 3000 ft"
In fact G-AB is still at crawling through 2300 ft, and was just reading back the reporting instruction. He's about to be hit by G-CD...
Overall, safety wins if reporting instructions are not read back.
As all ATC instructions have to be fail safe that would not be acceptable for issuing a further climb to another aircraft. The ATCO concerned would be going back to 'AB to confirm the situation. ( I do think however this is straying from the original thread).