Oops...I posted on your other thread in the England forum before I saw this one! My response is about Shetland. Good to know you've got ferry reservations from Aberdeen. DO follow Northlink Ferry on FB as they will put on there if there are delays. I have also seen that sometimes they go out early if they expect poor conditions.
https://community.ricksteves.com/travel-forum/england/must-visit-places-during-3-week-england-scotland-trip
Oh yea!! to Shetland! For myself 4 days (5 nights???) is not too much! I do think you are going to need to rent a car there. I did a tour there last year - Orkney and Shetland combined and did 6 nights on Orkney and 7 nights on Shetland and did not have enough time to see everything. We hiked a little bit but not much. We did take the ferry over to Yell, drove across and took the ferry to Unst...the Northernmost island in UK, saw the Northernmost bus shelter and ate at the Northernmost tea room for lunch, hahaha! A splendid day.
I've already booked with the same tour company for Shetland in 2025 and he's expanded it to 8 nights. I'll go up at least 3 nights ahead so I can do some things he doesn't cover.
I'm happy to give you links to the tour company's itinerary so you can see how he organizes days there. Here is the link to my TR but you'll need to scroll down to the start at the Shetland portion.
https://community.ricksteves.com/travel-forum/trip-reports/seymour-travels-scottish-island-tour-orkney-and-shetland
Are either of you interested in local textiles? Knitting, etc? We had a wonderful visit to the Hoswick Visitor Center. The ladies that run it are locals and have been knitting their whole lives, I think. Mark had arranged with them to give us a presentation but if you just stop in there they will talk away as long as a cruise ship isn't in port and they aren't busy!
We did 2 boat trips - one out to Mousa Broch, to see the Iron Age tower https://en.wikipedia.org/wiki/Broch_of_Mousa On another day we did one of the wildlife watching cruises out around the island of Bressay to see the Gannets and other birds. We were hoping for whales but saw none. We did see a school of porpoises which excited the boat captain who whipped out his camera with an extremely long lens so he could document them for the wildlife survey folks.
I'm not sure in your time frame is cruises will have started so you may have an advantage there. We only had a day or 2 of minor crowds with the cruise passengers.
Have a wonderful time. I'm glad you've got time on the end in Edinburgh