My dad had hernia surgery 4 days ago. He's having a lot of pain. The surgeon led him to think he wouldn't have much pain. Is this normal?

Yes. Hernia pain after surgery is very normal. It is highly variable from one person to the next, with some feeling much more pain. It usually starts to improve after 3 to 5 days, but can last longer for some.
Call your surgeon. There can be significant pain after hernia surgery but it should be getting better every day. If not, you should be seen and checked out by your surgeon.