Is using vias for split data buses a bad idea?
It will probably be fine in your case to via down to the bottom layer and have a small slot in the ground plane.  If you were running higher speed signals it could become a problem.

The issue to generally be concerned about with cutting a slot in the ground plane is that return currents will need to go around the slot, which can then act as an antenna and be an EMC problem.  Return currents are concentrated on the paths of lowest impedance.  For low frequencies, this is the path of lowest resistance.  For high frequencies, inductance starts to dominate the impedance, so the return currents will flow in the direction that minimizes the loop area.  This means that they will be concentrated directly underneath the signal trace until they reach the slot, then flow around to get back to the signal trace again. This will cause radiated energy and distort nearby signals (or farther nodes if they are high-impedance). With lower frequencies, the return currents will not even care to get close to the slot, though, instead making a diamond shape of lower current density centered on the slot.

That being said, 2-4 MHz is so slow I wouldn't worry about it.  If you're running at 500+ MHz with fast edge rates, though, slots in the ground plane should be avoided, especially underneath high speed data lines.