Question: Is not telling all of the truth a form of lying, in terms of breaking the Fourth Precept?
Answer: This depends on several factors. For instance, if there is legal declaration needed, such as in documentation or in court, the whole truth is usually expected. If what not declared leads to selfish advantages in terms of money and such involved, not only is it lying, it is also stealing, breaking of the Second Precept.
Such can be considered stealing from all the people in the country, if it is in terms of tax advantages, governmental subsidies and such, which might be harmful to others, even if the impact is subtly divided in effect for them as individuals. The negative karma created will be difficult to measure.
As another instance, if there is no obligation to tell the whole truth, such as when chatting with an acquaintance, for personal privacy and reasonable self-protection, it is not lying if overly inquisitive enquires are not fully replied to – as long as there is no deliberate misconception created for selfish benefit or to harm others.