I think the 4" screen would be ideal, as the 4.3" I tested on the DroidX was a little too big, and I've been getting frustrated at the 3.7" screen typing in portrait on my DINC (on many different keyboards). My iPod Touch is a wider screen (not as tall, but wider) and I can type better in portrait on that, so I think the 4" DINC S would be comparable portrait-wise.
I love the DINC, but I'd seriously consider upgrading just for the 4" screen...