Why my travel agent asked me to check in at airport at least 4 hours before the flight time?

I have pruchased Qatar Airways economy cl tickets for myself and my 2 toddlers. Before pruchasing I mentioned that I want front seats for more leg room and convenience and he said OK, but after I paid cash and he handed me the printed E-TICKETS (which itself is kind of strange for me) he told me to arrive at airport 4 hours ahead of flight time to make sure I get seats of my choice? What is this? What is going on? The last time I traveled was 3 years ago, that travel agent did not say such thing, rather I checked in only 1 hour before flight and I did get the seats which my ticket issuing agent had said he booked for me. Please help me to understand what is going on now.