1st catalytic convertors are covered under the 8/80 mile federal emissions warranty. 2nd, no way a cat costs $4K, I've replaced one on a honda accord, maybe $200 and 2 on my Z06, $600.
Third, any complaints you see on a forum should be taken both with a grain of salt and in context of where the complaint came from. This is a forum of for the most part "picky" or detailed enthusiast. Complaints are going to be concentrated here as some only came here for information and to make a complaint. Every ram or any other vehicle owner likely doesn't post on any forum so the million plus vehicles that have never had an issue, you'd never here from. In other words, you'll hear from owners with a problem 1000% more often than you'll here from an owner with no issues. Failed logic