I have a shoe horn some where around here... They get a little big. As long as you are willing to invest in the filtration for them then go for it.
Check on their natural habitat. I think they are a low current fish. When they grow they will be a little crowded. When that happens, it will be a reason for you to get another tank...bigger...better
I beleive a 75g is perfect for 4 adult discus....In my city, there's a breeder I know that's been dealing with Discus since the 70's....ALL of his breeder tanks are bare to the glass with the exception of a single brick....These breeders are 30g TALL tanks....Just one pair each, nothing more.....Most of his pairs go for $200-500, some $1000+.....So he's had some for many years making babies in the same 30g.....Please read upon these fish, I heard that their water perameters have to be tip top shape.....But, yeah, that 75 you have in mind will be just fine for 4 Discus.....Post pix....
Discus do better in a taller tank, so a 40 gallon breeder would be a little too shallow. The 75 would work fine, a 90 gallon would be ideal. with dicus come lots of water changes, do lots of research before getting discus. An adult discus will require 10 -15 gallons each so you could with 5 or 6 in a 75 gallon,