Hmm, so I went ahead and removed a link. All's well now with the rear brake - that was more than enough to offset those 4mm. On the other hand, the bolts have moved exactly half an inch forward in the track ends. Now that I think about it, the size of the cog and chainwheel shouldn't affect the predicted movement, which leaves me wondering why the rule of thumb isn't just 0.5 inches of axle movement per link removed, which is what happened in practice?