Would 3-4 hours be enough time to tour lego city or how much time should we plan for?

3-4 hours would be if you did everything and some things twice. Closer to the 2-3 range

Yes, that would be enough time, though my kids stayed even longer. They could play Lego all day

It depends aldo on the age and interests of the participants. We had one toddler who was too little for some of the rides. Another was afraid of the dark in another ride. Do the kids like target shooting? Can they occupy themselves with building, for example, cars and race them over and over? Do you plan to take a break to eat, then continue on? What time/day of the week are you going? Crowds can restrict your access to things or overwhelm people. Lots of factors to consider

Yes after a couple hours you will have seen and played at every station and then could go back and revisit favorites

Yes! 2-3 hours should be enough time. Everyone should wear gym shoes and pack light