I think generally a 4.1 is a good highway compromise, 4.4 (actually the 4.375) just lifts your 100kph revs so its a bit buzzy, but great fun through the twistys...
As Jason says, if you run bigger road tyres it can help, I ran a 4.625 in the roadster for a while with a 185/60/14 track tyre and a 185/75 road tyre to give almost 10% higher road speed and more suspension compliance...
To get an idea for cruising don't use 5th, as that's about a 15% overdrive (I think?!), so driving in 4th with your current diff would be like 5th with a 4.3 or thereabouts... obviously all the lower gears would be shorter too...