You don't say where you're starting from and if you only have 4 days a lot of that could be taken up travelling.
Are you camping or hostelling, airbnb or hotel? When do you want to go?
How remote do you want?
Most tourists stick to the main roads and stick together so you can usually lose them quite easily by venturing up a side road.
We found a deserted beach just a mile ot 2 from Arisaig on the West Coast to pitch our tent.
Loch Ossian youth hostel on Ranoch Moor is accessible by train, Corrour Station. Stunning scenery and pure air.
As has already been said, Dumfries and Galloway can feel a long way from civilisation.
I would probably avoid the Isle of Skye unless you have somewhere booked and you like crowds. I'm going there in August by bus, train and ferry but I have booked already to a part less touristy.