now I haven't done CrossTabs, but I would think that if you grouped your data by date, and that each group consisted of 13 weeks that something should be able to be worked out...
at least that is where I what I would try
Again, I don't have much experience with CrossTab style reports, and I don't know if my solutions would work or just give you 4 groups. It might require a subreport (which I hate doing but sometimes is the only solution)
perhaps someone else will have a better solution.
HTH