the May1989 Road&Track issue shows the following max speeds in each gear of a 190e2.6:
1: 27mph @5300rpm
2: 58mph @6400rpm
3: 90mph @6200rpm
you can then calculate your rpm in 4th gear for a given speed from the last figure above.
fyi, the power peak is at 5800rpm, and i guess that's the reasoning behind the 4th gear ratio.
i dunno why the max engine speeds are not the same in the lower gears though.