Just to follow up on my B-15 comment from the last thread, I know it's a cliche that the B-15 is arguably the best amp ever made, but it really is a Swiss Army Knife. A P bass (which the S4 clearly gets its lineage from) sounds amazing through one, and I find that the B-15 has been a great companion to my D4 as well.
Another suggestion is a Polytone. At big band rehearsals I tend to take an old Polytone, and while it's solid state, it also does well with both a P bass and the D4. Polytones are old workhorses for upright players, so it stands to reason it'd work for the D4 as I've found it to be.
The B-15 really just adds just enough warmth and magic. The Polytone is a touch more "clinical".
And for the record, my B-15 is a B-15N. I haven't played the D4 through my B-18 yet as the head is being brought back up to spec, but I contend it would work as well.
I talked to Rob on Friday. He called me to talk about the neck for my new Deep 4 before he sent it off to be glossed. (Yes, I'm having it glossed a la the Solid 4 ... so shoot me! I prefer my necks like that.) He was checking because it's the biggest neck he can build, and probably bigger than either of my other two basses. He's made. So if you think Rob just uses a single template - you're wrong there! I'm guessing it'll be between my current (and soon to be sold) D4 and the Takamine TB10, giving it more of a hybrid P/upright experience.