How much strain would 160 - 240 amp alternator put on a 4-cylinder engine?
It depends on the engine. Some 4-cylinder engines use better amp alternators than that. Basically, as long as your vehicle runs correctly, and you hook up an alternator that fits in the car and runs right, you shouldn’t have any problems at all.
Maybe bump the idle up a little bit. I’m assuming you’re not one of those guys that are running some gigantic stereo systems on the road and you need a big amp to run it. If so, you might also want to put a bigger battery in it, too. As long as the engine is running right, it uses a little bit more energy to charge system up, but realize that any alternator is only putting a full charge only when the vehicle needs it. If you have a big battery and a big alternator, it’s not always going to be putting up 240 amps; it will only give you the amps that you need. The rest just goes to ground so it doesn’t run all that power.