There's 2 routes you can go with Apple products. Option 1 is to buy the newest one every year/other year and sell the old one. Option 2 is to go all out and buy the least compromised device to last for 5 years.
Let's look at the potential bottlenecks you could run into after 4-5 years. (edit: this is comparing how it would fare vs 2017-2018 devices which I'm guessing will be quad-core/4gb RAM standard by then. I emphasize on the RAM because that's usually the biggest bottleneck in electronic devices if they don't ship with enough: see Safari or Infinity Blade crashing on iPad 1)
iPad: Non-retina, 256MB Ram, A4 single-core processor, heavy, thick, battery
iPad 2: Non-retina, 512mb Ram, battery
iPad 3/4: 1GB Ram, Heavy, thick, battery
With the iPad 3 and up, they put in Retina display and 1GB of Ram which is imo, the final pieces to make it "good enough" to last a few years. The A5X is a solid processor too I believe. Considering how many devices they put it in, I think it will be supported for long enough until you want to upgrade.