The Four Mile Trail was originally a "toll trail" built as a commercial venture. The first hotel in Yosemite Valley sat near the trailhead, so you follow generations of visitors on this historic trail. There are a few ways you can do this hike. The least effort in terms of walking, is to go to Yosemite Lodge and pick up the hiker's shuttle and have it deposit you at Glacier Point. From there, it is a leisurely 4.6-mile descent into Yosemite Valley. If you don't mind walking, though, it's ultimately a lot less effort just to start in the Valley and do the round trip. Thsi description assumes that you will do that.
The Four Mile Trail has one of the more accessible trailheads in the Valley. When it's running, you can take the new (June 2006) El Capitan shuttle and simply get off at the Four Mile Trailhead stop. Otherwise, you can drive the Valley Loop Road and stop in the often-crowded pullout in the shadow of Sentinel Rock. More enjoyable, though, is to avoid a vehicle altogether and start at the Lodge and just cross the Swinging Bridge and walk a hundred yards or so against traffic to get to the trailhead.
Even better, though, you can take the hiker's shuttle from the Lodge and hike the trail one-way downhill. This would give you the chance to hike to Taft Point and Sentinel Dome and then down the Four Mile Trail for about the same mileage as a round trip tour on the Four Mile Trail.
The trail goes straight back from the road a bit and then takes a sharp right at a jumble of boulders. Notice the white chalk marks left on these boulders by generations of rock climbers. Shortly, the trail begins to rise as you switchback up to the foot of Sentinel Rock and then make a long traverse underneath it. Once clear of Sentinel, you'll start a long series of swithcbacks with improving views at each level looking across to Yosemite Falls and down to El Capitan.
When you pass the second winter closer gate (the first being near the start), you are roughly half way there. At this point, the trail gets a bit less steep and eventually you start getting views toward Half Dome. Always present is Sentinel Rock, which allows you to measure your progress. You must gain enough altitude so that you are about even with the south (higher) summit of Sentinel Rock. At times you may appear to be gaining on it rather slowly, but stick it out. Once you're lookign down on the main summit of Sentinel, you're pretty much there.
Finally the trail flattens and traverses through a nice red fir stand with a bit of a meadow and a small drainage which can run rather late in the year. If you were to follow that drainage you would end up down at the Le Conte Memorial near Housekeeping Camp and that gully makes one of Yosemite's finer ski descents in winter.
Shortly, you reach the junction with the handicap trail out to Glacier Point. Crowds or no, the view from the point is always worth a look. If the gift shop is open, you can buy a drink and an ice cream to enjoy while you gaze out at Half Dome.