Greg - interesting. I think the better player bias is/has always been a part of this, since for folks who routinely hit greens in regulation, the short Par 4 challenges/subverts their expectations while still allowing them the fun (and ego boost) of another green in regulation. For lesser golfers, the short Par 4 allows for a rare green in regulation, plain and simple, and for the fun and ego boost that goes with that (even though there is little added complexity or challenges and no subverting of expectations in comparison to a regular Par 4.) In other words, everyone -- good and average players alike, if for different reasons -- love the short Par 4, and so it's pretty hard to mess one up/design a bad one. But precisely because it's so easy to design an average short Par 4, it's so hard to design a truly great one...and so for that reason alone I'm going to say that creating a great long Par 4 is easier that creating a short one. It's easier because no one really loves long Par 4s -- it usually gives neither the good player or the average one any satisfaction/boost, and so if the architect throws them even one little crumb, i.e. designs a relatively easy green to putt, or a wide enough opening for a run-up fairway wood, it suddenly seems like a great hole!
Peter