PeopleSoft portal isn't very popular and certainly won't survive into the Fusion applications. Most companies have pulled it out and implemented a more modern solution. I'm not sure now is the best time to pick it up, but if you really want to I guess you should read the PeopleBooks about it, figure out SSO, Pagelets, RSS Feeds, integration broker, etc.

If you've been doing a PS Admin job for four years, you're probably more qualified than me to know what tasks that role does. Off the top of my head, a PS Administrator does migrations, architecture, security (sometimes), compliance, coordination with other groups, database administration (usually), installations, upgrades, patches, etc.