Our DD had the same problem in 3rd grade, but it was a Montessori classroom so the math is a little different anyway. But they did just keep adding digits, and on top of that they insisted that she make all corrections to math errors before moving on (to the next digit, so no big incentive anyway). That was when we started trying different things outside of school--unfortunately they were not receptive to doing anything more for her at school, even though it was a not-cheap private school. Ultimately we did EPGY for awhile, but have since become quite enamored of AoPS and their free and quite catchy Alcumus problem sets. But it was all outside school--EPGY and Alcumus are online, so that would have been a stretch to do inside school anyway. This year the teacher is letting her work through an AoPS book on her own. Also we've been doing IMACS (live classes, in person) for a couple of years and she really loves those, but they seem to be limited to cities in the Southeastern U.S. Unfortunately, as discussed in a recent thread elsewhere on this site, a downside of trying to accelerate outside school is that the kid may end up having to take the same old math classes anyway (i.e., not being allowed to accelerate), so it's worth considering doing more 'enrichment' /lateral stuff--there is also a good article about this on the AoPS site by Richard Rusczyk (sp?), here: http://www.artofproblemsolving.com/Resources/articles.php?page=calculustrap

Best of luck!