I've went up and down the Four Mile Trail (which has been lengthened to about 4.5 miles). If you're in shape, it takes about 2 hours up if you're going strong, 3 at a leisurely pace. I've also hiked down the Panorama Trail then Mist Trail to Happy Isles. That takes most of the day, although it's leisurely and mostly downhill. Great close-up view of rarely-seen Illilouette Falls along the way. There's some uphill after Illilouette Creek up to the shoulder of Mt. Starr King. Then steeply down to join the John Muir Trail (horse trail).
Since you have limited time, I recommend taking a bus that leaves Yosemite Valley in the morning and heads to Glacier Point. Then start there. You want the "Glacier Point Tour" bus. Take the earliest bus, one way (currently $25 1-way for adults):
http://www.yosemitepark.com/Activities_ ... tTour.aspxReserve at any Valley hotel or by phone.
Here's short descriptions from the Park Service of suggested Valley hikes, including these:
http://www.nps.gov/yose/planyourvisit/valleyhikes.htm