I just read that most RC priests can expect four years of study at seminary after college… that seems like a lot of training, what exactly is their curriculum like?
Much of it is, of course, theology, i.e., studying the Catholic interpretation and doctrine of the Christian faith. This includes Bible exegesis, ethics and philosophy in general. It’s not just writings of Catholic philosophers - the doctrines of other writers, even of radical atheists and anticlerical authors, are also covered, but taught from a Catholic perspective.
At least in Germany, studying Catholic theology also comprises language classes - most specifically ancient Greek and Hebrew, the original languages of the Bible, although I think priests don’t have to achieve complete fluency in them, it’s more of a background knowledge.
Then you have the more institutional aspects, like learning about the governance of the Catholic church and its history.
Another big junk is of course liturgy - since the central duty of a priest is to perform religious rites according to the teaching of his faith, much of the curriculum is about the practicalities of performing Mass, giving sacraments, and the religious significance of these acts.
Here’s the webpage for the Masters of Divinity program at St. Patrick’s Seminary in California (the degree you need for ordination) laying out requirements: