It wouldn't be considered a four-function calculator if it were capable of more than four functions, but the term 'function' really only counts if the function would normally be found on a scientific or graphing calculator; including tax calculation functions would still classify it as a basic calculator, but including trigonometric functions would not.

As for the original question, the rules were written when there was a much larger divide between graphing calculators and computers, and one of the hallmarks of a computer is a full keyboard (as a computer is generally capable of word processing). PDAs were considered closer to computers, and certainly not education-oriented technology (in fact their purpose is mostly to store notes, something specifically avoided in testing environments), so those were also disallowed on exams.

The rule against full keyboards was to keep devices like the

HP LX200 out of exam rooms, since handheld computers with full 8086 processors and DOS were fairly popular around the time graphing calculators came about. Math exams are a test of the mathematical capabilities of the student, not their ability to perform data entry into a computer. Regulating the capabilities of calculators preserves this.

When the TI-92 was introduced, it was also TI's first calculator with a CAS, and TI marketed it as a product for schools which could not afford computers and math processing software for each student; it was a device to bridge the gap between computer and calculator. If you look at its capabilities compared to TI's other graphing calculators of 1995 (TI-80, TI-81, TI-82, TI-85), it's pretty clear they were well aware it wouldn't be accepted on standardized tests, and took advantage of this to offer some pretty slick features you'd normally need a computer for. The TI-89 came later following some miracles in miniaturization, and was intended to be a more portable TI-92 (+ Plus module) in a familiar form factor.