I have gone through a bit with best bass gear and Nordstrand as of late. I had the MM5.3 and sent it back for the 5.4. What the x.4 pick up is is a bit of a leap conceptually. Take a P bass pick up. Now make it in a single jazz bass pick up footprint. Then take two of these jazz bass style ones, and put them side by side. Then redo that in a MM foot print with the correct pole pieces and wind it to sound like a MM pick up. That is the quick explaination. The pick up will have five wires coming from it, White, Black, Green, and Red, with the exposed wire as a ground/shield wire. It wires just like a Seymour Duncan humbucker, and if you use it in a "single" coil split, the two coils that make up one of the sides of the humbucker are noise canceling. So 4 coils, made into two humbucking "single" sides of the MM humbuckers. If you want all of the wiring options, Series, Split, Parallel, you need to get a three way switch. A blade tele style switch for the classic Sting Ray look, or an On/On/On mini toggle switch for the modern splitting look. The hot and ground wires from this switch can go into your active preamp, or to the volume knob for a passive set up. Word on the street is that they are extremely nice sounding (great MM sound) and quiet in the single coil split mode. The MMx.3 is meant as a Direct replacement for the Sterling model of Musicman Basses, and that came from Brian at Best Bass Gear who was staying with Carey Nordstrand at NAMM, and asked that exact question. Brian was trying to help me out with some techno stuff and kindly asked Carey about this.
By the way, the customer service from Best Bass Gear is superb. Their prices are competitive or better than most, and they ship things extremely fast. I cannot recommend them enough.
Patrick