The cost-reduction ability of Johann's Stopgap doesn't change its mana cost or mana value, only the total cost you pay. Specifically, the mana value of Johann's Stopgap is always 4.

If the permanent is an illegal target as Johann's Stopgap tries to resolve, Johann's Stopgap is removed from the stack and you won't draw a card.

Bargain means "As an additional cost to cast this spell, you may sacrifice an artifact, enchantment, or token."

Bargain represents an optional additional cost. A spell cast with that additional cost paid is "bargained."

You may sacrifice only one artifact, enchantment, or token to pay a spell's bargain cost.

If you copy a bargained spell, the copy is also bargained. If a card or token enters the battlefield as a copy of a permanent that's already on the battlefield, the new permanent isn't bargained, even if the original was.

Some instant and sorcery spells require additional targets if they're bargained. You ignore those targeting requirements if those spells aren't bargained, and you can't bargain those spells unless you can choose the appropriate targets. On the other hand, you can bargain a permanent spell even if you won't be able to choose targets for an enters-the-battlefield ability of that permanent once the spell resolves.