What position on the memory gauge does "while you have 4 or less memory" refer to?

In what order do a [Security] effect, "when [...] performs a security check" effect, and "when a card is removed from [...] security stack" effect activate when they trigger simultaneously upon a security check?

P-194 [Aegiomon] performs a security check, and when it would be deleted in battle against a Security Digimon, <Barrier> prevents the deletion. At such times, can I activate this card's [All Turns] effect and digivolve P-194 [Aegiomon] into a Digimon card with [Aegiochusmon] in its name?