How do you know that they will only get four inches? You really ought to consider a different species. Even though they are relatively still fish, they still need space without being cramped against each other. Your tank (assuming it is the standard 20 long) is 30 inches long, each catfish is roughly 4-6 inches. This means that a single fish is 1/6 the length of your tank. These guys school. Say you have nine of them swimming together and three in a line, this line is now roughly 1/2 of your tank length. Then you still have thirty inches of fish next to them. Do you see the issue? If you go by the inch per gallon (not always reliable but a good start) you are looking at around 45 inches worth of glass cats alone. Even if you don't subscribe to the ipg guide, you still can get the idea that this number of fish of this size for this particular tank is not only unrealistic but is also quite cruel.