If you are truly thinking about "discipline" as a matter of showing your child how to behave, you will find that most "behavior problems" are problems of maturity rather than morality and that most of the problematic issues of discipline can be easily resolved. Some degree of attention-seeking behavior, for example, is a normal way for a very young child to respond to rationed attention from busy adults. If the ration of pleasant attention can be increased, he won't have to clamor to make you scold him.
Small children live in a world that's difficult for them to manage and in which they often stand accused of doing damage of one kind or another. Denying wrongdoing is therefore their most usual kind of lie and the kind that most often gets them into trouble. Your child breaks his sister's doll by mistake. Faced with it, he denies the whole incident. Your are probably angrier with him for the lie than you are about the breakage.
If you feel strongly that your child should own up when he has done something wrong, do make it easy. "This doll is broken. I wonder what happened?" is much more likely to enable him to say, "I broke it, I'm sorry" than "You've broken this doll, haven't you, you naughty, careless boy." But if your child does admit to something, of his accord or because you force it out of him, do make sure that you don't overwhelm him with anger and punishments. You cannot have it both ways. If you want him to tell you when he has done something wrong, you cannot also be furious with him. If you are furious, he would be foolish to tell you next time, wouldn't he?
Tall stories get some children into trouble too. In early childhood, a lot of children still confuse reality and fantasy, and what they wish had happened with what really did. After all, they can happily accept stories about the Easter bunny while keeping a quite unmagic rabbit of their own; they see no conflict between the two.
If you are going to read your child stories and help him to enjoy the mythology of childhood in his culture, such as Santa Claus, it is unreasonable to jump on him for lying when he comes in from a walk with an elaborate story of his own. Of course, he didn't really meet a space lady. He probably doesn't even think he did. But just as it's sad if older children force a four-year-old to pick holes in the fantasy of Santa Claus ("We haven't even got a chimney, silly"), so it's a pity if his own fantasies are beyond the pale. Enjoy the story. Being not true does not make it a lie in any moral sense.
Parents sometimes worry because their children seem to have no regard for the truth at all. They may overhear them mentioning Mommy's new dress when she hasn't got one, or announcing that they were sick last night when they weren't, or just telling a friend that they are going out for lunch when they aren't. There are lots of reasons for casually inaccurate talk and an important one is that the child hears it from adults. Adults tell endless untruths out of tact, kindness, a desire to avoid hurting other people's feelings or to save their own time. Children hear them. Your child hears you agreeing with Mrs. Smith that the weather is much too hot when you have just told him how much you like the heat, hears you on the telephone excusing yourself from something because you have invisible visitors. Unless the reasons for these "white" lies are explained to him, he cannot be expected to see why he must never exaggerate or falsify when you can.
If your child tells so many stories and adds so much embroidery to his accounts of daily life that you cannot be sure what is true and what is not, it may be time to make it clear to him why truth matters. Don't fall back on it being "naughty" to tell lies. Instead, try him with the story of "the boy who cried wolf." It is a good story. He will enjoy it. Having told it, you can discuss it with him. Point out that you, and all the people who help take care of him, really need to be able to distinguish between what is true and what is not, so as to be sure of knowing when something important has happened to him or when he is really feeling ill or scared. Phrase the whole conversation so that he feels you care about his telling the truth because you care about him and want to be sure you look after him properly, that it is a matter of accurate communication rather than "being good."