Should I extract teeth while wearing Braces?

I went to the ortho, he said that I would have to extract 4 premolar teeth ( 2 top and 2 bottom ). He also said that when I remove the braces later, I would have small gaps between the extracted space of about 3-4mm. So now, he put the expander and going to put the braces on next week. I would have to extract the teeth when wearing braces if the situation needs it to be. What should I do? Should I extract, and would 3 -4 mm gaps be very visible? I don't want any gaps. Thank you for your help.