I would consider the fourth precept on musavada (lying) in Buddha Dhamma to include all abuses done by speech. That would also include harsh speech, slander, and gossip which will harm oneself and others. When one truly understands Buddha Dhamma, i.e., the nature of this world as embodied in anicca, dukkha, anatta, one sees that these precepts come out naturally. At that stage, one’s mind automatically rejects all ten immoral actions (dasa akusala) and thus the five precepts are automatically obeyed.
Of these 10 immoral actions, the four vaci sankhara (immoral acts done with speech) include, musavada (Lying), pisunavaca (slandering), parusavaca (harsh speech), and sampappalapa (frivolous talk). When it come to a lay person these are not promises but one’s intention is to do the utmost (otherwise the act will itself be a musavada or a lie). There comes a time in one’s personal practice when one realizes that there is no other moral way to live.
Precepts are about purifying one’s mind. A pure mind gains wisdom, and will not allow any harmful action by speech or by deed. Such a mind is not burdened, but has “cooled down”. People could blindly follow precepts, but are of greed, hate, or ignorance. Then depending on the state of their minds, they may get reborn in a lesser world.
If keeping the precepts alone will take one to a higher plane of existence, then a cow or a horse living in isolation will be certain to be born a Deva.. They do not kill, steal, lie, or get intoxicated, and if their owners do not have any other animal of that kind, then there is no chance of sexually misbehaving either.