Just from curiosity, clicked your name and saw that you'd done a lot of independent travel in Portugal, Spain, italy & South of France, always in August, and your postings indicated you'd done homework about those destinations. That's why it surprised me to see you'd picked the 2 most over-touricized isles, and at a time when both will be not just thronged but Heaving with jillions of tourists (with added influx weekdays of up to 3 cruise ships daily, adding 8,000+ cruise crowds lurching around following the Guide with the Umbrella). There are ways to strategize around this, but you should plan with this knowledge... and it Does involve using some candid guides, not just promotional websites. For both the isles and Athens, and Greece in general, I recommend ROUGH GUIDE to GREECE (any edition -- you can pick up cheap on Amazon, & pull out separate "guide-ettes" on destinations, language, food, history etc). On the 2 isles this website is straightforward: http://www.greekisland.co.uk/santorini/santorini.htm
ITINERARY: for such a short visit, the most time-efficient strategy is, immediately on arrival, FLY to farthest destination, then take ferry to #2, leave Athens for LAST 3 days. That way, no panic about a last-minute glitch in transport. If your homebound flight is at a sane hour (9 AM, in-line no earlier than 7AM), your last night can be enjoyed in Central Athens, & taxi in AM to airport. If really early, then there are 3-4 places to stay near the airport. I suggest fly to Santorini, fast-ferry to Mykonos, then fly/ferry to Mainland.
SANTORINI: Yes, 4 days may be too much;try 3. Re your Oia-Fira conflicts, there's a 3rd Answer that works best of all! Firostephani, also right on the same fab "rim view". It's the northern fringe of Fira. NO bus tours/cruise crowds stop here ... so even at Height of hi season it's relaxed day & Night, AND it's walkable to/from the Fira Nightlife... nice flagstoned path parallel to main road. Also, a handy bus stop on Main Road to get to Oia (so u don't have to go to the Fira-town Zoo of a bus-yard). U can take early AM bus, 8 or 8:30, and have a couple hours in Oia before the throngs pour in... then leave & do something else -- wine tour? Akrotiri ruins? Your choice. It will be hothothot, the beaches are on the non-view side of the island, and gritty, black, volcanic... so you'll want a pool. FIrostephani pool-hotels are still available if your budget permits.
MYKONOS: Again, that UK website is straightforward: http://www.greekisland.co.uk/mykonos/mykonos.htm Mykonos in August is all about party-time with beach-boozers starting in afternoon, plus clubs at night. Beaches can be VERY windy -- not sheltered from fierce "meltemi" (umbrellas are thatched, set in concrete footings). It's at least as expensive as Santorini, maybe more. You might want to consider an alternative -- NAXOS (my choice) or PAROS. These isles have much more to offer (ruins, scenery) AND the plus that you can ALSO see Mykonos... there's an excursion vessel 2 days a week. In August, Naxos nightlife in Port Town is vibrant, several busy clubs, good music bars -- and you can stay at upscale hotels at beach just adjoining the port town walkable to/fro. In addition, a car rental (or bus) can take you to far beaches, just comfortably busy but not packed ... and inland from beaches there are mountain villages, Hills of Marble, farms, ancient ruins etc. Another plus: NO cruise ships, no charter-flight package-hol crowds (airport takes only smaller planes).
ATHENS: Here I say, find hotel close to Acropolis, easy walk to all sights. I like the SOUTH side of Acrop, just off the scenic no-cars pedestrian walkway. Look at Hotel Herodion or Phillipos, high floors, for balconies w. view. Mid-August the moon rises right at sunset, and on Full Moon Night (8/18) the famous landmarks (Acropolis, Roman Forum, etc etc) are open late & free, for musical & dance performances... a magical night.