I thought you were replacing "regular" crits.They wouldn't; it would be a regular crit.
On a regular hit, the dagger does 1d4 damage. If the player rolls a 4, it does 4 damage... it doesn't explode because it's not a crit.
On a critical hit, the dagger deals 2d4 damage and thus, has two opportunities to explode. If the player is lucky, he rolls a 4 and gets to add another d4 roll to the damage.
If the player is super lucky, he rolls two 4s, adds 2 more d4s to the damage, and gets 2 more opportunities to explode.
I understand now. Thanks!