At around 4 ish, I took my daughter out of the Marathon and bought her another 5-pt harness - the Graco Nautilus.
Please see this video about the importance of keeping your children in 5-point harness car seats as long as possible.
http://www.youtube.com/watch?v=azgBhZfcqaQ
I bought my daughter a Graco Nautilus (similar, but less expensive than the Britax Frontier.
If you want a bigger, seat that is similar to the Graco full back booster, look at this one:
Sunshine Monterey - no 5 point harness, but I think it offers WAY more protection than a flimsy Graco full back booster.
http://sunshinekidsbaby.com/?OVRAW=sunshine%20monterey&am...
My son is 9 now. When he was around 5, I purchased a Britax Husky (the biggest, safest 5-point at the time). He was in it till he was 8!
I hated having to put him in the booster bottom, but he sits in it now because he's not quite ready not to be.