When my son was that age, we ended up switching to a HBB for travel. We did it the first time last year right around his 5th birthday. He was a smidge over 40" and maybe 40 lbs soaking wet. We used a graco turbo booster and it worked out very well. It was nice not having to install two car seats by myself (I also had a 10 month old and an 8 year old and I was traveling alone with them.). At home DS1 (now almost 6) rides harnessed in a frontier, which I love. We do let him ride in the HBB in DHs car and for travel. I knew he would be better off in a harness, but I couldn't fathom traveling with the frontier (it's a beast) with three kids alone.
DD (3/06)
DS1 (7/09)
DS2 (8/13)