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With four weeks you can pretty much see all of the country. I'd suggest at a very brief canter:
A week or so based in Edinburgh from where you can also do day trips by train to Glasgow, Stirling, Fife.
A drive of the Northern Loop - essentially up the east coast through Fife/Perthshire/Grampian to Inverness/Loch Ness the north eastern highlands and then possibly take the ferry over to Orkney.
Then across the North and down the lovely West Coast, stopping at or travelling through (not exclusively) Torridon, Ullapool, Bealach na Ba, Plockton, Kyle of Lochalsh/Skye, then south through Fort William and Glen Coe
On to Oban/Loch Lomond and then south through Ayrshire before working your way back north through the Dumfries/Borders and all those lovely Abbeys and rolling hills to Edinburgh.
Of course that's a lot of driving, but over three weeks you could break it up very nicely. Just to give you an idea of timescales I did Edinburgh and around the northen loop over a four day weekend one Easter, so while it is some distance it can be done in reasonable time.
If you give us more of an idea of your interests I'm sure you'll be flooded with more detailed suggestions!
The one thing that occurs to me is being here at the end of October may mean short days and bad weather - not the best conditions for driving, so my suggested itinerary may be a touch ambitious!