I would be all for Kasdan as the writer for Indy IV. I consider ROTLA to be a perfect movie all around. Great acting, fantastic setting, etc. And where does all that stem from? Amazing writing. You can have tons of great parts, but if the writing is poor you will wind up with a bad movie. Sure, a good script which is poorly acted can also make a bad movie, but there's a better chance it will be rated as at least OK. I have seen several Kevin Costner movies which are, at worst, OK, and he's not really that great of an actor. However, he has worked with some great scripts. OK, I'm going off on a tangent here - back to the point...
Kasdan would be a great choice (IMO). He's very familiar with the character and the series. He has also proven he is more than capable of doing justice to an Indy script.