We went twice last year with a 3.5yrd old and 1.5yr old. We used car seats for both on the plane. The kids, while they didn't say, preferred their own seats. They slept for most of the flight in the comfy confines as seats they are used to. A bit of an additional hassle but we used the stroller to wheel the car seats around the terminal, boarded the plane first to get the seats in and everything was great. We have an Eddie Bauer car seats and a Britax roundabout.

I missed the post where the OP said the 4 year old is only 34 pounds... stay with a 5 point harness, absolutely! Much too little for any booster.