Like most, love the Indy Trilogy, and I think they ended it pretty well with "The Last Crusade". If Spielburg/Lucas/Ford think they might have something great, than go for it, but I get the feeling it's just another way for each of them to prove themselves they still got something left.
Also, Harrison Ford said recently in a interview (don't know exactly where) that he is absolutely sick and tired of answering "So, when are you doing Indy 4?" questions, and just wants to get the whole damn thing over with.