Four?! Why not more?

[Kostas Kritsilas]

Like Steve Simpkin said, if you want a bigger/deeper stack, get an RPL machine.

In the earlier days, transistor count was critical as it would determine if a chip, and by extension, a calculator could be built. Later on, as chip fabrication improved and more complex chips could be produced economically, you could get units with bigger/steeper stacks. I think it has been proven, that aside from the most complex of equations, that a 4 level stack is adequate.

I also believe that you could emulate a deeper stack by using registers, or by writing programs to do so on the programmable machines (at least some of them).