Going on past practice, I would not expect the iPad mini 2 to disappear, but it will be heavily discounted if there is a mini 4 announced. When they announced the mini 3, the 2 immediately dropped $100 in price in most stores. It seemed like a no-brainer at the time, given that it had the same specs as the 3, and I bought it. The situation this time won't be comparable, as the 4 will get a processor upgrade. But if it is discounted another $75, as the original mini was when the 3 came out, it would still be a good buy. The A7 chip in it is plenty fast.
20" iMac 2.66 ghz Core 2 duo, 2009 & 2012 mac minis and 2013 13" MacBook Air