"¿Tiene usted una mesa para cuatro?"
Translation:Do you have a table for four?
Para is used here because the people receive the table. If you said "por cuatro", it would be read like you want to buy the table in exchange for four... Pesos, probably.
How can I get it wrong for stating "you all" but it's correct in another lesson?
Duo accepts "you all" for "ustedes," the plural form, not "usted," the singular.
Rule of the thumb: if you wouldn't address someone on a first-name basis, use the usted form.
Not a matter of politeness. It's just more common to invert the subject-verb order in questions where the subject is a personal pronoun. "¿Usted tiene...?" sounds a bit more like "And you, do you have...?", putting more emphasis on the person.