What next for 4yo outgrowing Radian 80?

[beltm]

Ds just turned four, is 40 lbs., and 42 inches tall with a very long torso. He is on the top set of slots on the Radian 80 with a little room left, and the top of his head is just about at the top of the at depending on how he is positioned. He still falls asleep often in the car seat and while he's not squirmy back there, I don't know if he's ready for a lap belt yet. What do we move him to next? Just a high back booster like graco turbo, britax parkway or Dino Monterey? Or do we get a harnessed booster combo like a britax frontier or graco nautilus? He's not a thick stocky kid, just very tall. He mostly rides with me so I am prepared to pay for a comfortable safe seat for my car. He hardly rides with dh, and it is mostly for short trips, so I may not need the most expensive seat for his car. I have latch in mine, dh does not have latch.

Also what do we travel/fly with? In the past we've folded the radian 80 in the bag and checked it. Are there any folding boosters that? I love they'd idea of the bubble bum booster, but don't think he's ready for backless yet since he falls asleep easily in the car.

What would be the next seat for him and what would be a good fit given that he has a long torso and is tall for his age. I NEVER thought we'd outgrow the radian at age four when i bought it. Granted we bought it when ds was four months as he had outgrown the graco snugride so maybe I can't believe it. I would like to keep ds in the five point harness as long as i can as I know it's safer. I haven't had to look for a seat in four years and I'm a bit overwhelmed. Any input is greatly appreciated, thanks!