If an Elan Aegis obtains the Unlocked Talent and Access Psionics Powers, can they stack fortify, lesser +1 and fortify +2 and Resistance (due to Elan) +4 on saving throws (spending Power points)?
Psionic powers stack in exactly the same ways as magic spells: typed bonuses do not stack with other bonuses of the same type,1 and untyped bonuses do not stack when applied repeatedly by the same effect.
Fortify and lesser fortify both supply resistance bonuses to saving throws. Since both bonuses are “resistance bonuses,” they do not stack. They also don’t stack with the resistance spell, or with a cloak of resistance, since those also supply “resistance bonuses.”
The elan’s resistance ability, despite the name, does not supply a “resistance bonus.” Instead, it is a “racial bonus.” This does stack with fortify or lesser fortify (or resistance or a cloak of resistance), so you could get +5 or +6 that way. And, per 1, racial bonuses stack with themselves, the resistance ability will stack with almost everything—the only thing it won’t stack with is itself, so you can’t use it twice2 to get a +8 bonus.
Unless they are circumstance bonuses, dodge bonuses, or racial bonuses.
It is possible to use the elan’s resistance ability twice before the first expires, by using it once on your turn, as a swift action, and then again after your turn ends, as an immediate action. There is no benefit for doing so, since it is a supernatural ability and therefore cannot be dispelled.