8 AM arrival one should plan on approximately 60 minutes for immigration, baggage claim and customs. It could take as few as 30 minutes, but only if you happen to be "first plane in" of the North America bank of flights. It could (but is unlikely to take) longer than 60 minutes.
If you are flying domestic from EZE, you'll still need to go deal with your luggage, possible check-in, and getting to Terminal C. I've never done that, so I don't know how long that will take, but it has to be no less than a half hour.
If you are flying Int'l/Int'l and your bags are checked through, 2 hours is fine. There's easy transit security, and then you're back in Departures.
4 hours to AEP is probably fine. I wouldn't cut it any closer. And I wouldn't cut EZE any closer than 3 hours for anything else than an Int'l/Int'l connection with bags checked through.