It is different, the lobe angles are slightly different than a 6BT. We're talking fractions of degrees. Irregardless, the 4B, known internally as the B3.9, began design phase in 1980, entered production in 1983. The 4BT came out a couple years later with simply a turbocharger added and the compression ratio modified slightly. The 6BT didn't start design until 1982, entered production in 1984 in industrial application. Therefore the the rumor that a 4B crank is simply a B5.9T, aka 6B crank with lobes cut off and not timed is completely incorrect.
