Why wasn't Sean Connery in the last Indiana Jones film?

Because he hasn’t aged as much as Harrison in the last 20 or what ever years.

And his character was already pretty old in the last one. The fourth took place about fifteen years later, didn’t it? It’s not hard imaging Indy’s father being dead (which he apparently was).

Bingo MrMeltedC. I assume he would be long gone by the time frame that the fourth movie takes place in.

Because he doesn’t want to make movies anymore:
http://www.dailymail.co.uk/tvshowbiz/article-357694/Connery-fed-Hollywood-idiots.html

His character was dead. Indy was supposed to be “too old” for all of that, but if his father was around being even older it would have been harder to take all of the age references from Indy.

He probably read the script.

Nobody else did.