hi, its good that your 1st 2tb-to-4tb upgrade was successful
ideally all the rest will be too but can i check how long you waited after the 5 green lights, and if the 2nd 4tb drive was of the same model as the 1st?
it may be a case where the 2nd 4tb drive came loose, which prompted the message (but when you put in the 2tb, it saw it as a fresh drive and started rebuilding again).
usually a rebuild will take about 1 day per 1tb of data that you have and will probably also need about 31 hours to complete. (if ive done the sums correctly, based on this it seems as though you only have up to about 1.5tb of data?)
best thing is to let the drives rebuild completely, and to let things settle a bit, and to do a full reboot of computer and drobo to make sure all boots fine again following the rebuild.
btw can you remember if your drobo-s is using Single (sdr) or Dual (ddr) drive redundancy?