dalremnei wrote: ↑Thu Nov 04, 2021 6:15 pm

Textbook input calculators internally have a stack much deeper than four levels, so, no.

Your considerations are more than 40 years behind the reality. Texas Instruments wanted to show off with the large "AOS stack" in the 1970th (9 levels in brackets + 8 open operations) in the TI 58/59 manual, the corresponding example in the manual only required a 3 level stack in RPN.

Even if some trains have a few hundreds of wheels, my car don't need more then 4.

For almost all typical calculations in schools and universities, 4 stack levels are not a restriction - on the contrary, this can help with structured work. For users who want more stack levels, HP also has RPL aka "Entry RPN" (big stack) alongside the classic RPN for around 35 years, but it has never been more popular then classic RPN.

In 2021, HP will be offering 5 RPN pocket calculators, but only one of them with Entry RPN. Even at Swissmicros, with more than 10 different RPN calculators, there is only one with Entry RPN and that even after-market. I think it's nice that there are such calculators for people who like or need it, but I think it's "special interest" and even worse for newbies to get started with because it can confuse how to understand and solve a calculation.

My programmable calculators - former: CBM PR100, HP41CV, HP28S, HP11C - current: HP48G(256kB), HP35S, Prime, DM41X, DM42