There are so many places it can be overwhelming.
I would recommend that you look into Aviemore and Rothiemurchus area. There was a good walk that some of my walking group did, but I didn't. They walked partway into the Lairig Ghru and then veered off and went over some boulders and back to the loch. Here's a link to Lairig Ghru walk that is too long for what you want. If you are interested in this area, pm me and I'll get more details from our guide.
Glen Affric is a gorgeous area. There is a walk you can do in a day, but it's longer than your 2-4 hours. It's the Loch Affric Circuit walk listed here. If you want something much shorter and easier, just walk in a while on the dirt road. It's a beautiful place.
I love the walks above Ullapooll. The Ullapool Hill Walk described in this list is very nice. The views of Loch Broom and the Summer Isles are quite lovely. And in August the heather is fantastic.
You really won't go wrong with Torridon. Here's a link to the walks. I think that this is the walk that we did.