Rookie questions on booster seats

[Mommy2Abby]

Hi - hoping to get some advice from you knowlegeable folks....

DD turned 4 in January, and she is about 40 inches (with a long torso) yet only about 33 pounds.

We have a Britax Boulevard in my car, and Britax Marathon in both DH's and Grandma's car. All are installed in the center of the backseats. She still seems to fit fine in them, except the seats just seem to sit "high up" especially in my car (Audi A4) -- and it seems more evident now that she is getting taller. Is that normal? (Obviously, she is still well within the weight and heigh limits for both seats....)

Should we wait until she is 40 pounds to switch her to a booster? And then what is the best booster for a skinny kid?

When we do switch to a booster, is center installation still the safest for a booster?

TIA!

[Joolsplus2]

Harnessing as long as possible, up to age 7, is preferred, so I'd keep using your current seats till her shoulders are AT the top slots. You can look at lower-profile combination seats, like the Britax Frontier or Graco Nautilus, that will harness her now and work as boosters in the future (at least 5 or 6 is when kids are safe in boosters, and can sit still and can buckle themselves easily). Any harness is equally safe, you won't be stepping down in safety to go from a Marathon to a Frontier/Nautilus.
Center is still safest as long as the booster fits nicely there (headrest doesn't push it way forward, or the hump in the middle doesn't make the seat tip too much). Hope that helps (I remember putting my own kid that size in a booster years ago, and it was so much harder to buckle him--all the leaning over the booster seat!--, and there was so much reminding him to sit still that I got him a harnessed seat again because it was easier for ME, but now there's recommendations to keep them harnessed so much longer, I'm glad to know it was safer for HIM :))