Can I use Smite the Monstrous on creatures given increased power/toughness?
if the enchantment is exiled/countered, the creature would revert to the 3/3.


This sentence is ambiguous. If the Enchantment spell was countered, the creature was never 4/4 in the first place. If the Enchantment was destroyed while enchanting the creature, then the creature would revert to 3/3. Let's go through both situations:


When your opponent plays the enchantment card it goes to the stack.
At this time it's a spell, not an enchantment.
You then have a chance to respond to it.
In this case, you need a counterspell to counter the enchantment spell. (Something that says "target spell")
(You can't cast Smite the Monstrous because the creature is still 3/3 at this time.)
If you do, the spell is then countered.


The creature was never 4/4 because the enchantment spell was countered. It never became an Enchantment and never did its thing.


Alternatively, you can let the spell resolve. It then becomes an Enchantment and will make the creature 4/4.
At a later time, when you have priority to play spells, you can cast Smite the Monstrous. Since the creature is 4/4, it is destroyed.  


It doesn't matter if a bonus to power/toughness is temporary; it's 4/4 at this time. The whole point of playing the Enchantment was to give the creature +1/+1. You can't just pick and choose when it benefits you. Sometimes it does, sometimes it doesn't.
(If I had a dime for every time a player tried to do that...)