E.S.
She is too little to be in a booster (seatbelt positioner).
She needs to a bare minimum of 40lbs. and if your little girl is like mine those 4 lbs. could take more than a year to be gained.
Are you worried that she has outgrown the Marathon height wise?
She has not: http://www.britaxusa.com/car-seats/marathon/grow-with-britax
Keep her in the Marathon until she has outgrown it either height or weight wise. You could easily get 2 more years out of this seat.
Keeping her in a 5pt. harness for as long as possible is the best.
My DD is 5.5, 45lbs., 50" tall and still harnessed in her Britax Frontier.
HTH!