Four full days is enough for Mainland unless you're going to hike every trail or dive Scapa Flow. A half day is enough for any other island except Stronsay which is worth a full day. The problem is the time to get back and forth to the more distant islands.
Adding other islands (other than the string down the causeway) will take careful planning since there are days the ferries don't run one route or another.
One really good walk is out to Mull Point - - the path, such as it is, runs along the North Sea cliffs, starts at a little parking lot by a blowhole, and isn't more than a couple miles round trip. Do it in the afternoon so the glaring sun isn't between you and the cliff edge or else you'll have to move back from the edge and miss the best scenery. If the wind is gusty or strong out of the west, skip the walk unless you have a buddy to hold the rope around your waist.
Kirkwall would be the place to stay since it has more places to eat and is at the hub rather than out at the far end of a stick. Nothing would be more than an hour away. I'd stay as close to the Front as possible so you wouldn't have to drive to supper.
To get there, maybe spend the night in Thurso, then scoot down the hill to Scrabster for the first ferry to Stromness. NorthLink runs that one. I think it's something like Orkney Ferries that makes the short runs between the islands. I don't know who makes the run from down around Gills out to Saint Margaret's Hope. I'd get reservations for the two long hauls. The other trips are just back-and-forth frequent shuttles, mostly (Hoy, Rousay, Wyre, Shapinsay). There's also a run from Aberdeen to Kirkwall, but I can't imagine how long that would take, nor can I remember what the livery was on the vessels. The long-haul ferries have adequate cafeteria meals, the smaller ones just drinks and snacks.
Depending on time available, I'd spend a couple nights just outside the park in Pitlochry since it's central to a few other things and pretty darn fine in its own right. It's also a fair base for exploring the western side of the park. When it was time to get into the central area of the park, I'd enter at first light, driving up Loch Dee and opening the thermos at the top to watch the sun come up.
Ballater is a pretty good place to stay for a couple of nights in the central area, but you can run out of places to eat real fast. Aviemore is a congested mess, I don't even swing in there for coffee anymore. If you decide to drive the Spey Vally, it's more enjoyable to use the small roads than sucking diesel fumes along the A9/A95.