My son will be 5 next month, is 42" and 34 lbs. His car seat was rated for 40" and 40 lbs so I had to get a new one b/c he outgrew the height. He is in no way ready for a lap harness booster, even a highback one. He is still much too slim, squirmy and small for that. A 5-point harness is the safest for as long as you can.
To that end, we replaced his seat with a convertible booster (chose the Britax Frontier and a comparable Evenflo for my husband's car). They have a 5-point harness for 57"/85 lbs and 52"/65 lbs respectively. They should last us in this configuration until he is in first grade (the Britax longer). At that point, they will convert to highback belt positioning boosters.
Some of his friends already use the little backless boosters b/c it is allowed by law. However, I don't feel they are nearly as safe, especially at this age and size, so even if he pulls the "all my friends are doing it" routine, it doesn't matter. He will use the safest configuration until he no longer can!