Not specific to the 40 or your question of the III vs IV, we have found at least one circumstance where the combo of card speed and especially buffer write-speed can be a serious factor. Among our shooting contracts are a variety of events ranging from weddings to dramatic productions and athletic events. When you shoot a long, buffer-filling series of shots at the end of a card's capacity you have to wait for the buffer to completely download to the card before changing cards to resume shooting. Waits can seem interminable as the action continues to unfold, but your camera is effively locked up.
We have to counter that in two ways:
-Stop shooting and change cards before they are absolutely full if critical action is looming.
-Carry a second camera body with a lens having an overlapping zoom range with the first.
Huge buffers are a bonus most of the time, providing the camera allows you to add shots before the buffer is completely empty (which older bodies such as the Fuji S2 did not). But a huge buffer is not a bonus at the end of the card if the camera is slow to download it completely. While card speed is only a component of that process, you will certainly notice its role when you switch from a newer, faster card to an older one. As a result we're retiring older, slower cards as quickly as we can replace them with the fastest new cards, and meanwhile identifying buffer write speed as a critical criteria in the choice of our next bodies.