M.,
SO GLAD you mean a HARNESSED BOOSTER SEAT - so many parents are putting their little 4 year olds in boosters too early.....
Either Graco Nautilus...you can use the 5p harness until child outgrows the harness/weight should make it to 5 or 6, Britax Regent (has very high weight 80 lbs and high harness slots) or the new Britax Frontier (also high weight and then turns into a high back booster) it is coming out soon, someone mentioned the Safeguard Go..my friends daughter and others I've heard find it uncomfortable...I have the Apex 65 but don't love it, it needs high seat backs and has short straps, it was 99.00 and is used as a spare (you get what you pay for)...personally I would get the Nautilus or Frontier but luckily my almost 5 year old is still fitting great in her Marathon. I've heard from some Britax devotees that the Nautilus does pretty well but the straps aren't twist proof like the Britax ones...the Frontier looks great and has lots of cover choice too and is FAA approved.
B.