Obi-Wan sees the message from Princess Leia in Ep 4. Obi-Wan assumes Vader is chasing Leia. Obi-Wan needs to use Luke to defeat Vader. So truth is probably not the best motivator. "You must learn the ways of the force if you're to come with me to Alderaan and kill your dad."
Alternatively, nothing in the message from Leia would indicate that Vader is on the scene at all. Maybe Obi-Wan wouldn't have even accepted the mission had he anticipated encountering Vader. No reason to say anything to Luke about daddy. Obi-Wan has no idea if Vader is even still alive until they confront each other on the Death Star.
On the other hand, Obi-Wan SHOULD have said, "Luke, I know this might come as a shock but the hologram is your twin sister. You must learn the ways of the force if you're to come with me to Alderaan and rescue her." Now that's a motivator!
So you asked the wrong question. :) Why did Obi-Wan not tell Luke the truth about Leia in Ep 4?
He wanted to protect him from the truth. Because if he knew that, Luke would be very sad and mad if he know the truth and shocked. But maybe he wanted to let Luke find the truth by him self.But maybe it would be better if Obi-Wan had tell Luke the truth.
If some old hermit I hardly ever see took me into his house thingy, told me about Jedis, light sabers, and the Force and all of that stuff about how it surrounds us, yadda yadda yadda, it penetrates us, yadda yadda yadda, etc. and then said , "Oh yeah, by the way, your dad is an important, evil, murderous Jedi tyrant in the Empire. I was his master. Sooo, we cool?" I would have said,"Ummm, A: you are crazy and B: what?" It would freak me out and it definitely would have freaked him out.