I have almost nothing to go on... I vaguely remember the villain's name starting with a k, and maybe something about a skull. The villain's name might also start with an s, and have a k in the name, and be what's making me think of a skull. At the end of the book the main character, who is either a man or boy, has to disguise his thoughts about somebody because the villain is asking him about them, and can sense his thoughts or read his mind. It seems like he pictures a girl riding somewhere on a horse. I'm pretty sure there were some sci-fi elements as well as fantasy. I think it was a series, and that the book I'm thinking of may have been the fourth one. For some reason I thought the books were by Ursula k leguin, but none of them seem to match. I have a vague memory of the villain being called a warlock, or a warlord? And something about a bird...either he could he change into a bird, or use a bird to spy on people. Not completely sure about that but that's all I've got!
It could be the last book of David Eddings' The Belgariad series, Enchanters End Game - It fills a number of your requirements:
- The villain's name is Kal Torak
- The main character, an upper teens boy, is trying to confuse the villain (who is trying to enter his mind) with thoughts of his (future) wife - who, in other scenes, is riding a horse.
- This is the 5th book out of 5, not the 4th...but close.
- One of the other main characters will often shapeshift into the the form of an owl.
Even if it isn't what you were thinking of - I recommend the series, they're fun and easy fantasy reads.