Four?! Why not more?

[Steve Simpkin]

Of course if you want to have an RPN (not RPL) calculator with a stack limited only by memory, you can turn on the Big Stack option in Thomas Okken"s amazing Free42 (V3.0 or newer).

"Free42 3.0 introduces the option of using a dynamic stack, also known as Big Stack. In this mode, the stack can grow arbitrarily deep, limited only by available memory. This is similar to the stack in the HP-28/48/49/50 series calculators."

https://thomasokken.com/free42/#doc