I think that when it comes to a film like Indy 4 the job of the marketing department isn’t so much to sell the movie as it is to not “unsell” it. There are already millions of people who know Indiana Jones and who are pre-sold on going to see the new movie. However, many of these people are going to keep an eye on the marketing looking for signs that the magic is gone. Is Harrison too old? Does the concept still work? Is this just an attempt to cash in on a beloved character? So the main focus of the marketing has to be not confirming people’s worst fears.
This poster works very well in that sense, largely because it doesn’t show us much. Just a box and a few recognizable elements (the hat, the whip, “Dr. Jones”). It’s enough to get us to remember the old films we loved so much and to get the nostalgic juices flowing, so to say. And the box adds an element of curiosity since it gets people to ask themselves what could possibly be in there, which makes the whole a poster even more effective.
As time goes by they will have to show a little bit more of the movie. But for now the approach of showing as little as possible seems like a good one.