There are many reasons why Indy IV will be better then any PT movies:
1. Spielberg is directing it, and he rarely puts out duds. Not all his movies are great, but he doesn't just put out some shit movie unless the story is bad. Example, Jurassic Park II, its not a bad movie, its just the story is so outrageous once they get to America with the Dinosaurs, it turns into Godzilla.
2. Indiana Jones movies were never made for kids like ROTJ and the PT tried to go after. There won't be as much kiddy shit in an Indy movie cause Lucas isn't going after that market. There won't be Jar Jar or the Ewoks, cause Indy movies are targeted towards teenagers, like SW & ESB were.
3. Lucas isn't financing this movie like he does with SW, so he is not constantly thinking, "How many people will I get in the theater if I do this or that?" He purposely made TPM for 5 year olds in some respects because he wanted to get a whole new generation of fans in case the OT fans grew up and didn't care, I think he knows that the main demographic for Indy fans, will be mainly Indy fans from 81-89.