Under what circumstances can Carrion Grub trigger Garruk's Uprising?
Your guess is correct; in the first case Garruk's Uprising will not trigger; in the second case it will.
Because Carrion Grub's mill ability is a triggered enters-the-battlefield ability, it doesn't do anything until it is too late for it to help with Garruk's Uprising. First a creature with power 0 enters the battlefield, and then later that creature grows into a 4-power creature. It never happened that a 4-power creature entered the battlefield. It is similar to a land that is on the battlefield later becoming a creature; you don't get &quot;when a creature enters the battlefield&quot; effects.
In the second case, Garruk's Uprising will trigger because Carrion Grub's first ability is a static ability, it is always true while Carrion Grub is on the battlefield. The moment it enters the battlefield, the game checks to see what happened, and it will see that there is a creature on the battlefield with 4 power that wasn't on the battlefield before.

603.6b Continuous effects that modify characteristics of a permanent do so the moment the permanent is on the battlefield (and not before then). The permanent is never on the battlefield with its unmodified characteristics. Continuous effects don’t apply before the permanent is on the battlefield, however (see rule 603.6d).


603.6d Some permanents have text that reads “[This permanent] enters the battlefield with . . . ,” “As [this permanent] enters the battlefield . . . ,” “[This permanent] enters the battlefield as . . . ,” or “[This permanent] enters the battlefield tapped.” Such text is a static ability—not a triggered ability—whose effect occurs as part of the event that puts the permanent onto the battlefield.