Use the same amount of roux that you would normally use. 4 Tbls, heaping or not, seems far too little. I usually use at least a full jar in a 3 quart pot. But, it really depends on how much roux you like in gumbo.
And, while I know many are snobby about the roux in a jar, it's the exact same thing you would make on your own stove so it's just as good as making it yourself. I mean, it's just flower and oil. There is really no difference in buying it in a jar or making it on your own stove. It's just a lot easier and faster. And it means you can make gumbo more often. That being said, I wouldn't use those dry roux mixes. I have no idea what is in those.