It's been one a year since the first came out.
The quality has really gone down though. Mario Party had a ton of games that were pure luck (Shy-Guy games). Why even bother dressing up what is obviously several different ways to flip a coin? And don't bother playing computer unless you team up. The random games always seem to go in their favor.
I hope 5 shows a turn in the series, because 3 was so awful I skipped over 4.