I don't like any booster seat for a 4yr old. He should still be in a 5pt harness car seat, there are plenty on the market that will allow him to remain in a harness for quite some time. The Graco Nautilus is $150, will harness to 65lbs and then becomes a booster and no back booster to 100lbs. This seat will last him at least another 2-3yrs in the harness, then becomes a good booster seat. You will get your moneys worth! The Britax Frontier will harness to 85lbs and then boosters to 120lbs. Its $300, but again worth the mone yas it has a 9yr life span.
If you insist on a booster seat, please make sure that he is not only 4yrs old, but at least 40" tall and 40lbs. If he doesn't meet even one of these requirements, he should still be harnessed (and shoudl anyway at 4). A small child has a great risk of internal damage called SEATBELT SYNDROME, because their hips are not square like an adults hips, they are rounded and cannot hold the lap belt down in a crash. The lap belt should ride low on the hips, not up on the belly, but smaller kids just don't have the body to do this properly. He should also always sit in a HIGH BACK booster seat. No back booster seats provide zero side impact protection, and a small body like that of a 4yr old NEEDs that support in even a minor collision.
If he still sleeps in the car, he needs a harness. If he still reaches for things or slouches in the car, he needs a harness. If he's under 40lbs he needs a harness.
Each step up in car seats (from rear facing to forward facing, harness to booster, booster to belt only) is a major step down in safety. Please consider keeping him in a 5pt harness.
A forward facing car seat is outgrown when one of three things happens; the child goes over the weight limit, the ears become level with the seat back, or the shoulders go over the top harness slot.