I dont think it is easy to get jump to PR #8 from PR #4 within six months unless you practice black hat SEO/ Linkbaiting etc.
It's always good to have ambition but PR8 in the next update from PR4 isn't achievable unless you can get good quality PR8+ links now (and even then it would take a minor miracle)....don't happen to know Bill Gates or any of the Google founders do you? Aim for PR5/6 and you may get there....good luck!