Exactly what Amy said. The job of a booster seat is just that, to boost the child up so the shoulder belt lays across the shoulder and chest, and so the lap belt lays across the hips instead of the abdomen.
If your child is only 4yrs old, ESPECIALLY if she is under 40lbs and 40'' tall, please keep her in a 5pt harness. Even if she is right around 40lbs, you can pick up the Graco Nautilus for $150 at Wal Mart. It will harness to 65lbs, then becomes a high back booster, and eventually a no back booster, to 100lbs. My 6.5yr old is 50lbs and 52'' tall and can still use this seat.... he is still in a 5pt harness as well, the Britax Regent.
If you want to know why he's still harnessed, please send me a message and I'll let you know about the near miss that made me stop at Babies R Us one afternoon and purchase the Regent for him.
Keep her harnessed, buti f you do choose a booster for her, please choose one with a high back. A high back IS better than a no back (someone said they were the same, they are NOT) as a high back will offer side impact for her head in a collision, instead of her flying to the side she will be contained in the headrest of the high back booster. The Compass is a good booster. Remember that she needs to be mature enough to not sleep in the car, stay sitting up straight at all times, not put the belt behind her shoulder, and that YOU still hook her seatbelt to ensure that its clipped properly, pulled up tight, and laying in the correct spot.