I would guess because it didn't sell well. The 4004 was supposed to be a "Modern" Rickenbacker bass. The 4003 is essentially a 4001 with a few updates. The 4004 had a few problems, more 4004 basses had neck problems than 4003 basses for some reason, and the ill-fated 4004cii5 bass was a problem with pickups that did not cover 5 strings. Also the toggle, volume, tone wiring was a poor choice. Many 4004 owners rewired their 4004 to VVT.
I own 2 4004 basses, I put single coil pickups in both, one is wired with a toggle and 2 stacked pots so it functions like a 4003. My other 4004 has a 4 band audere preamp. I converted my 2 4004 basses to 5 string basses. The 4004 neck is actually quite similar to the new 4003s5 basses.
The slab body and rounded edges make the 4004 very comfortable.