Firstly, the four year minimum contract starts after you have had your second and final checkout.
Secondly, I definitely wouldn't mention your overall grand plan to become a pilot in the interview if you get that far. The interviewers are not going to warm to someone who plans to leave the company as soon as he can, especially with the shortage of staff we have at the moment.
Thirdly, I also wanted to become a pilot, but I must say that after working for three years as a controller I am perfectly happy as I am. You can always go flying in your spare time, and ten minutes after my shift has finished I'm back at home with my feet up and not stuck in some hotel. The money issue doesn't really come into it either, not that it would be a motivation to becoming a pilot for most people.
Good luck anyway