seat belt vs harness

[lisavarma]

Hi there,
My daughter is 40 lbs and 4.5 yrs old, so I am looking into getting her a booster seat. I was thinking of getting the Nautilus.

Should I keep her in the harness or do I use just the seat belt?

I am assuming that the harness part is for up to 65 lbs and then after that I just use the shoulder/lap belt?

thanks,
lisa