Four?! Why not more?

[Steve Simpkin]

There were some practical reasons to only have a 4 level stack. The first two generations of HP RPN desktop calculators only had a 3 level stack. Adding more levels would have required more hardware (magnetic core or semiconductor) which was expensive. The HP-35 added a 4th level that was shared as a temporary register during trig operations. The 4th level, and the single memory register, was likely judged to be enough to solve most problems and the number of transistors inside the ICs (around 6,000) was already pushing the limits of available technology. Also with only a single level display, keeping track of more than 4 levels would have been deemed harder. This last factor (and tradition) probably kept HP's later calculator models at 4-levels. The addition of a Last X register starting with the HP-45 also helped.

When memory became cheaper still and multi-line displays became available, RPN evolved into RPL with its stack size only limited by memory.