FYI, a floating shield is useless.
A few things I can think of.
- You get 4V of noise, how are your scope probes setup? Are they set to 1x or 10x? (It looks like you have the same scope as one I use at work, and I know they can be either). If they are set to 10x, try setting them to 1x and see if the problem is still 4V. (Looking at the problem can make it worse since the probe is high impedance)
- Try the low-pass filter. Since you aren't going for anything high speed, Use a big cap and small resistor. Make sure to put it next to the arduino.
- Try adding some clip-on ferrite beads near your arduino on the cables. Just clip them around the whole cable.
- How are your grounds configured? Do you have a loop in it, or is it in a proper star configuration? Can you put the electronics in a box, separate from your motors?