"Standard" roof slopes are 4/12, 8/12 and 12/12. Given you are not
dealing with snow loads, I would think all of these would be excessive
for your application. Maybe 6"? 3-4" is going to be pretty flat. Not
that the extra 2" is going to even be a noticable visual difference.
How much rain are you talking about? 1/2" over 2 hours at a time or
2"/hr. A light rain will be more forgiving of the flat roof, but long,
hard rains will want a steeper roof to facilitate water removal.
Just my thoughts. Worth exactly what you paid for 'em.

JW