Are you sure it said 4-8 weeks? Every email about a new certification I've ever received from ISC2 said the welcome pack would take 8-12 weeks. While emails about certificates for renewed certifications have stated 4-6 weeks.
ISC(2) is a small shop so timing may take a bit longer during certain times of the year. My understanding is the end of the calendar year through the first quarter have historically been a bit slower than the rest of the year. Reason being is that more people sit for the exam December through February from what anecdotal evidence I have seen and heard over the many years.