Hi Bill3075,
Welcome to Flyertalk!
Will you have checked in for the CO flight in ATH or will you have to do it at the transit desk at T4 once you have arrived at T4 after taking the airside flight connections bus? ( and what is the minimum check in time for CO?)
Also are you on one ticket ( protected in the event of delays) or two tickets ( not necessarily protected).
I would feel uncomfortable about a 90min connection from T5 onto another airline at T4 ( esp transatlantic) as you only need a small delay to miss your miminum check in time for CO ( if not already done so).
Can you get an earlier BA flight from ATH to T5?
Regards
TBS