I would go with six ga for that run, though I agree 4ga is also an option.
The bigger the wire the better the wire the wire is bigger when the GA is small
For a 30' run the price difference should not be all that bad
Option 2: Go to the electrical supply and take their advice.
Note on options 2: If you pay the store, and it turns out they gave you bad advice.. Some might argue they owe you $$$$.. Where as here, Well, At most I owe you what you paid me ($0.00) so your Free is hereby cheerfully refunded :)