Just adding resistance to ground when the switch is in the series position won't really be a controllable attenuation. It will simply damp the treble then suddenly kill the volume all together. Nor will just adding some in-line resistance. This will sort of work a little better but the attenuation you get will vary from one amp/pedal/DI to the other, completely dependent on its input impedence. You need two resistors - one in-line, and one to earth, forming a voltage divider. The resistors can be individual fixed resistors, usually around the same value, as this gives 6dB attenuation which is usually about right for this. I've wired this many times over the years. The two resistors can be two halves of a pot as well. I've never tried this, but I'd think a 200K trimmer pot would be ideal.
You'll need a 4 pole switch. I think if you went with a couple of fixed resistors instead of a trimpot, you could get away with 3 poles, and have your four positions, but really, I wouldn't bother with a 4 position switch. The difference in tone between one single coil and the other is negligible. And you can get a 4P3T for just
a few bucks on ebay. I'm lucky, [Invalid or Expired Link Removed]
Here's a wiring for a MM triple coil that I often use. If you have trouble adapting it for your use, I might get time tomorrow to scribble out a specific diagram for you...
If you decide to go with a pair of fixed resistors and your bass is passive, instead of 68K, go with larger resistors - 100K I'd think.
PS: FWIW, notice that the dummy coil is in series with the bridge coil when in single coil mode. This is a really nice way to do a triple-coil MM pickup...
