Pages '09 does not mean that we didn't receive updates last year or the year before that etc.
Office 2011 for Mac requirements suggest 1GB of memory (beyond the base requirements for the Operating System). Office 365 Home Premium for the Mac bumps that to 2GB. As long as you remain at OS X 10.7.5 or earlier, a 4GB machine would just squeak by for either Microsoft solution. However, if your workflow requires a browser session in concurrency with other open application windows, or large documents, the 4GB would quickly become anemic.