My iPhone 4 is slow. That's not ACTUALLY a surprise. There was a time where I was on an upgrade treadmill with my PC. A new video card here, a new processor there. Then a full MB swap, more RAM...every year, something else would get replaced. Progress marches forward.
PCs eventually reached a bit of a plateau. Unless you're playing really intense games, you're not going to notice that your machine is old and slow. A four year old PC does most of the basic tasks asked of it, because those tasks aren't terribly hard anymore, and you've already got a lot of RAM and a 2GHz CPU.
But mobile devices are just starting to reach that plateau. Putting more RAM in a phone makes a difference, but they haven't been loaded up from the start because of size and power restraints. Every year sees a small advance in battery tech and low-power computing. So my old iPhone 4 is well behind that curve. That's how things go.
A four year old Android phone is going to have the same issues, assuming we can put aside the question of whether it's getting updates at all.
This is one of those cases where I don't think the manufacturers have a particularly malicious intent. My iPhone 4 is slower compared to the day I first got it, but it does SO much more, and it does those things a lot better than it used to. My experience is richer, even if I have to wait an extra second or two for certain tasks to complete.