A lot depends on how much room they are going to have for a run and if they will be confined for extended periods.
3' high minimum. About 4 sq ft of floor space per duck but if they are going to be inside for long periods, heavy rains, freezing or extremely high temps, 8 sq ft per duck. You don't have where you live so that will depend on the materials you use. If you are in a temperate climate, you need an open air coop with a wind break, if you live in the north you need an enclosed coop with adequate ventilation.
So 4 ducks, I would build a 32 sq ft coop so, 6' X 6' will give you space for nests, feed and water. This will probably not be enough room if you have more than 1 drake. Outside they need 10-25 sq ft per bird, the bigger the better and easier to clean.
The coop needs to be secure yet provide adequate ventilation. Many people use dog houses (though 6x6 is kinda big for that). What you use to build with will depend on the predators in your area. Safest would be 2 X 4's and hardware cloth.
If you have chickens, something similar to their coop but you don't need roosts.