It is my understanding that the primary value of internal job boards is twofold: (1) they can help increase the speed with which job-openings are filled; (2) they can help decrease employees' beliefs that positions are being filled, unfairly, based on politics rather than merit. But listing new positions, internally, for more than 2 weeks before going "public" defeats (item 1) -- especially if the internal pool of candidates is not as strong as you need, at present.
Unlike my colleague, above, I believe that requiring all new job openings to be posted IS good practice -- especially because of (item 2). If employees feel that current hiring processes lack transparency, it's incumbent upon you to increase said transparency. (And send a message to hiring managers who may not have been sufficiently transparent up until now, as well.)
Giving your employees a "heads-up" as to the jobs being posted sends another important message to employees, even if they're not qualified for the openings -- especially if they're not qualified for the openings. Why? Because these are the jobs of the future for your organization. And if employees want to be part of that future, they will now be able to see what competencies are important for them to develop. Should you then be able to promote some employees who took it upon themselves to focus their attention on learning such competencies is a wonderful story to tell, far and wide.
But 4 weeks too long to wait before looking outside.