Yup, Andy. While some of the units would read correctly from a 4GB card, most of them could not write correctly to it, so at a minimum, it would bomb trying to write the ttgo.bif if the system were on the card, and later, things like the mapsettings file if there was a map loaded on the card. I even tried a 4GB non-SD in my old 720 before they had gotten past the version 7.221 navcore and it just wouldn't cut it.
It was interesting at the time because we all first thought things were working correctly with those cards because we could get the units to see the map(s) on the card and such. In actual use, we were running into trouble. 4GB non-SD cards were aberrations in any case as there was technically no SD specification for such a thing. Before SDHC finally came around, a number of companies played around with these, but many devices didn't have the proper internal drivers to read/write them correctly.
TomTom was a real oddball in that it appeared that the Linux USB drivers for reading were able to handle 4GB, but not for writing.