"We were told they had approx 4563 applications last year".
I'm not involved in the world of ATC but don't let that figure put you off applying or dent your confidence.
4563 would have been filtered down to 1000-2000 genuine applicants/contenders very quickly if the candidate pool was in any way representative of the general population. Sadly, intellectual and behavioural incompetence is widespread in today's society and most people aren't good self-assessors of their capabilities. I don't envy the good folks at P&C who have to assess several thousand online applications every year.
The recruitment team would have rejected a significant percentage of candidates' online applications due to their comprehension and written communications skills being way below the required standard. Similarly, online aptitude testing would scrub out those with below average brain power. (i.e. those who have to use their phone's calculator to add up to the price of their takeaway sandwich and whom stopped paying attention in english and science midway through high school).
The selection process would knock out all the hopefuls with below average intelligence in the initial stages so don't flip out at the metric that only 1% of applicants make it all the way to an ATC tower or centre.