As the OP is a "working" bassist playing covers I think giving 5 string a try is a smart move. If you can, get something that's on par with your 4 in terms of quality, and make sure your rig is suitable to produce the additional low notes. Playing 5 string on a bass or through an amp that is lacking is frustrating and you don't need any additional frustration when you're trying to learn a somewhat "new" instrument.
I picked up a used 2010 P5 back in late October and it's the only bass I've gigged with since (I'm in a cover band BTW). Love it.