There was an RS2 Road, and then an RS4. The ATB crank after the RS2 was the RS3. The spiders from the RS3 will fit and RS4 cranks (I have just checked) so I assume the reverse is true.
The difference between the RS3 and 4 is the crank arm shape, the RS3 looks pretty much the same as the current RS7, where as the RS4 crank arms are more or less straight, like the RS2.