With only 4 days, it will be a problem. As other answers have said, Scotland is a big place.
If you're flying into Edinburgh, I would recommend spending those 4 days doing a thorough job of exploring Edinburgh. There is a lot of history in the city, so it will keep you plenty busy enough.
For hiking, climbing Arthur's Seat is an iconic part of a trip to Edinburgh, and will occupy you for most of a day if you wander round the crags around it and Duddingston. (And allowing for a pub stop before you return!) Walking around Holyrood House and gardens could easily take another day, as could the castle, depending on your interest in history. And the city itself is hilly enough that you will be kept fit just walking around.
If you really want to do some more strenuous hiking, I would recommend the last section of the West Highland Way to get a feel for the Highlands. If you get the train to Tyndrum, that gives you four days of the recommended route. The route follows the old Military Road, now a wide gravel track, so it is almost impossible to get lost and the path is easy to walk, although you do still have substantial height gains to deal with. If you're in good shape then you should be able to add Ben Nevis to the walk for a long day's hike; or you could get the train up to Bridge of Orchy instead and cut some of the distance.
If you're hiking in Scotland, do be aware that the weather can change dramatically and unpredictably. I once hiked up Ben Lomond with my parents, whilst my sister stayed in the campsite around 5km away. We had continuous horizontal rain for around 3 hours, all the way up and down. My sister at the campsite was sunbathing in a gentle breeze and full sunshine, with barely even a cloud. Whatever the weather seems to be like, you need to pack full waterproofs (jacket and trousers) and a warm sweater or fleece. This doesn't relate to travelling, but it's an important note if you're hiking, especially if you're used to hiking somewhere like central France with relatively stable weather. Most people who need rescuing in the hills, it's because they get cold (and usually wet) and hypothermia shuts their body down.