MB4s were on the more obscure and exclusive end of the Wal catalogue and were only available for a limited period of time. That said, over that short period from around 1990 they gained a reputation for being just about the only midi-bass that actually worked.
That was largely down to the genius electronics provided by Australian Steve Chick. The stroke of genius was to remove the need for the bass to midi converter to rely on a hexaphonic pickup to sense the pitch of the note being sounded. The beauty of a keyboard synthesiser is that the keys are effectively just switches. As soon as they are "on" they are "on" - there is no delay in getting them engaged. Hex pickups, on the other hand, take a few cycles of the note to sense what note is being played. For the loser notes on the bass that could potentially take more than a tenth of a second. Definitely noticeable for the listener, positively off putting for the player. Add to that the potential that overtones in the note could be interpreted as an entirely wrong note. For this reason midi basses gained a reputation for being tricky to play and badly glitchy. At the very least they demanded a very precise playing style.