The only things that are relevent

That the flight was delayed
That a misconnect occurred
That arrival at final destination was > 4 hours behind schedule
That the delay was due to a problem with the aeroplane. No need to spend time trying to find out why the problem occurred; if the airline wants to deny the claim in court, the onus is on the airline to show that compensation would not be due rather than for you to prove it is

Based on this information, it would seem that you are entitled to EUR600 compensation from AY

Write a letter to AY with the pertinent information with a request for the compensation due under EU261