It's probably not base four because you have to stretch out more pairs to match up four pairs and that's entropically disfavored. However ribosomes can accomod

It's probably not base four because you have to stretch out more pairs to match up four pairs and that's entropically disfavored. However ribosomes can accomodate a four pair matching, though at a very reduced yield (unless you think Schultz's postdoc fabricated those data)