Generally, effects do not stack if they're from the same source. Don't blame me for Pathfinder's inconsistent use of the verb stack. Blame 3.5.
The feat Psionic Talent taken 4 times grants a total of +5 PP
When a creature gains a benefit from a source, unless specifically stated, the creature doesn't combine the benefits if the creature attempts to gain that same benefit from that same source. Instead, the creature gains only the largest benefit.
- Energy Resistance: If a source grants a creature fire resistance 10, and the same source later grants fire resistance 20, the creature reduces fire damage they'd take by 20 points per instance.
- Movement Modes: If a source grants a creature a fly speed of 60 ft. and the same source later grants a fly speed 30 ft., the creature can fly at a speed of 60 ft.
- Temporary Hit Points: If a source grants a creature 5 temporary hp, and the same source later grants 3 temporary hp, the creature has both 3 and 5 temporary hp, and up to 5 points of damage apply to the creature's temporary hit point pool, and should the 5 temporary hit points wear off while 3 or more of those remain, the creature has 3 temporary hit points, else they have as many temporary hit points as they had when the 5 temporary hit points wore off.
To let the the feat Psionic Talent grant an increasing number of power points, the feat says two very specific things and phrases those things so they work using this concept. First, a creature can take the feat Psionic Talent more than once. Second, each additional time after the first a creature takes the feat, the benefit from the feat Psionic Talent increases by 1.
For perspective, if a feat granted fire resistance 1 or a Speed of fly 5 ft., and such a feat could be taken multiple times, the feat would--to be absolutely accurate and true to the underlying mechanics--pretty much have to say the same thing as the feat Psionic Talent does to avoid using the verb stack, which is supposed to be used exclusively for bonuses and penalties. Dungeons & Dragons 3.5 also failed in this regard, with its Extra Foo feats for paladins, barbarians and others.
This same linguistic mine field was successfully navigated by the D&D 3.5 feat Font of Inspiration, yet folks still fight about that feat. There are probably even fights about the Pathfinder feat Godless Healing, but I couldn't find any.
The weird phrasing is a necessary yet confusing and ugly way of doing things because of the way sources and numbers-that-aren't-bonuses-and-penalties interact in Dungeons and Dragons 3.X and, by extension, Pathfinder.
- The first time a creature takes the feat Psionic Talent, the feat grants the creature 2 PP.
- The second time the creature takes the feat Psionic Talent, the second iteration of the feat Psionic Talent grants the creature 3 PP, replacing the benefit of the first iteration.
- The third time the creature takes the feat Psionic Talent, the third iteration of the feat Psionic Talent grants the creature 4 PP, replacing the benefits of the first and second iterations.
- The fourth time the creature takes the feat Psionic Talent, the fourth iteration of the feat Psionic Talent grants the creature 5 PP, replacing the benefits of the first, second, and third iterations.
"O, Then This Feat Sucks!"
As Matthew Najmon succinctly put it earlier, "There are plenty of [Pathfinder] feats with completely unambiguous effects that are every bit as crappy as gaining 1 power point."