Before my time - almost!
So I had to resort to the Blue Book. As you've probably guessed, these are all updates, through time. The 4100 and 4150are be different models though. Let's take the 4150 as an example. The 4150 itself dates from 1959 (I was only 2 then!) and has a pretty basic sort of spec. It runs until 1962, when the 4150B arrives, and this adds reverb, 'slide' (copying the Lowrey 'glide', of course), and the Spectra-Tone. 1963 sees the introduction of the 4150BD, which adds repeat (reiteration) for the upper manual voices and a two-speed Spectra-Tone. The updates for the 4100 are on similar lines. These seem to be the final updates to these models and soon after then, the tubes gave way to transistors.