First statement 'If you have not heard a response within 4 weeks...' came from Aer Lingus.
Second statement 'All applicants should expect to hear response...' came from 3rd party on-line tests supplier.
I'd be inclined to assume only successful candidates will hear from HR. This seems to be the standard practice now due to high application volumes and the 'shamrock hiring' thread implies some calls have already been made. Out of the estimated 5,000 applications made you would assume someone would have been told they had been unsuccessful by now if the intention was to contact all applicants.
I imagine they are incredibly busy and may still be contacting people after the deadline of the 8th. Also historic AL threads seem to suggest calls continued to be made 2-3 months after closing date so stay hopeful.
Probably a team of 2-3 people have been given the unenviable task of wading through 2,500-5,000 applications. This is at the same time as dealing with the Cadet scheme assessments. A very busy time for them so be patient.
My advice - keep watching, study up, be ready and keep your fingers crossed.