We (3 physically fit
adults) will be in Scotland for 6 days in mid-April. We will spend the first 2 days in Edinburgh, and the last night in Edinburgh, which will leave us with 4 days to travel somewhere else, beginning and ending in Edinburgh within those 4 days. We have narrowed choices down to the following:
Loch Lomond and The Trossachs
Kintyre peninsula
Colonsay
Arran
Oban/Iona/Mull
Could anyone advise us as to which places we should choose? Also advise us as to whether we need to book places to stay ahead of time or whether we might maintain a bit of spontaneity and find places as we go? We are looking especially for natural beauty and places to walk and hike. We are not choosing Isle of Skye because one of our party has already been there.

OK -- first of all - on such a short visit, splitting your Edinburgh time into two parts means an extra check in/out/hotel change which is not very efficient. In a perfect world you'd want all you Edinburgh time at the front end or at the end - not split. Are you flying in long haul or will you have already been in the UK or Europe?

If your 'mid April' Dates include the Friday through Monday over Easter you definitely will want to pre-book.

Any of those could work. With four full days you could actually squeeze in both the Trossachs and Kintyre. However, the Trossachs could fill 4 days very easily. (Stirling, Doune, Inchmahome, Loch Lomond, Killin/Falls of Dochart, up into Perthshire, Lochs Tay and Earn, and a day trip up yo Glencon/Glen Etive). That would tick all your boxes

But first work out how to get all your Edinburgh nights together -- If you are flying out of EDI you could stay 3 night in Edinburgh, then collect a car and visit the Trossachs (or which ever region you decide on) - ten driving from a base in/near say Callander you'd be less than an hour's drive from the airport.

We will be flying into/out of Edinburgh, which is why we are splitting up our time there. We arrive Easter weekend and have already booked an Airbnb in Edinburgh for the weekend, departing Easter Monday. We fly out of Edinburgh the morning of the following Friday, so we will travel somewhere else Monday-Thursday, so we can sleep in Edinburgh Thursday night and go to the airport after breakfast on Friday.

With that info -- definitely try to put all your Edinburgh nights at the front end. If you base in the Torssachs you would be an easy drive to EDI on Friday

Stirling = 45 mins
Callander = about an hour
Aberfoyle = about 1.25 hour

Or if you want to be closer to EDI - at least spend ALL day Thursday out exploring and drive to the airport after dinner that evening, turn in the car and stay at a hotel atEDI. There are at least 4 hotels right at the airport

Presumably you're aware that mid-April is likely to be wet throughout the west, and the Highlands in general. Not trying to second guess you (it's your trip after all) but have you considered alternatives on the east side of the country, where the odds are (slightly) better you'd have drier weather?

Otherwise I'd probably agree with Janis that the Trossachs might be preferable, since staying in that area you'd have a fairly wide range of alternative day trip destinations, which you could pick and choose based on real time weather forecasts.

I agree with Gardyloo that at that time of year the Borders/Fife/Aberdeenshire might at least be drier.

That was another reason that just from your wish list I recommended the Trossachs - being away from the west coast gives you more of a fighting chance

April could be anything from nice to wintry and anything in between and I wouldn't want to plan an itinerary around any of the islands

Orkney?

I was hoping that someone could help me choose between Isle of Arran and Isles of Mull and Iona. I have read that both are beautiful, and I think I can go to Loch Lomond and the Trossachs en route to either. We really want to see beautiful beaches. We don't have a lot of time, and it seems that going to the Western Islands makes more sense than Orkney, given this limitation.

I know Iona has some beautiful beaches. You do know they aren’t for swimmers unless you’re a native or a polar bear. What do you mean as to beautiful. I prefer the small coves with cliffs rising on both sides along the Cornwall coast, not everyone would agree with me.

You only have 4 days. Not nearly enough time to 'do' the Trossachs en route anywhere. You can drive through the,but that's about it. Driv g in be rural bits and especially on the islands is very slow. 30-35 mph on the mainland and 2,5-30 mph n the islands. As I mentioned, I personally would not pre plan an island-centric itinerary in April. I have been to every place on your list - some like Mull multiple times, and the Trossachs countless times for as much as aa week at a time - I'm givi g you my best advice, but it is your trip so your choice

Western Islands tend to have a few more "god botherers" though.

Have not been to Arran, but can heartily recommend Mull (which we added to our trip thanks to these here Fodorites, sound advice as usual). Tobermory itself was a fabulous little village.

Here's the blog from our trip, including photos, if you're so inclined to check it out: https://stricklandia.wordpress.com/

Rain in Scotland can happen in any month - April or August it doesn't care. You don't go to Scotland for the weather, but you do dress for the weather once you are there.
Does it have to be an island you visit?
Consider also the lochs of Argyllshire if you have to have water, or even going up the less visited but still beautiful East Coast from Edinburgh. Or up to Cairngorm for some wilderness.
Whether you can do it all in four days depends on what you are wanting to do really.

Thank you all for your advice. Now I am thinking of choosing a location between Oban and The Trossachs/Loch Lomond to stay for 3 nights and from there visiting Mull/Iona/Staffa as well as Loch Lomond and theTrossachs. Does that seem sensible and doable?

I would go with Oban; if the weather's okay the outings to Mull and Iona are easy, or there are plenty of places to visit on the mainland for shore days.