I think our growing season is so short that by the time it's gone to seed, there'd be no time to grow a new plant but even if that's not so, the police don't want people to call about their neighbours' pot plants. I'm pretty sure no-one would even notice if it was just one or two extras...but I dunno. I'm guessing confiscation and a ticket.
If you had significantly more than four plants, possession ftp trafficking maybe?
There must be some legislation surrounding all that but I haven't seen it yet.