This is the exact backpack trip I did two years ago. As already mentioned, day one from Horseshoe Meadows to Solder Lake will be a long day, about 12.5 miles. But going this route gets New Army Pass out of the way the very first day, while you're still fresh. On day 2, once you get to Lower Rock Creek crossing, the climb up Guyot Pass is tough, but this day is not quite as long as day one, about 10.5 miles. Day 3 is a piece of cake to Guitar Lake. Just be sure to stop off at Timberline Lake on your way up. No camping is allowed here, but it is a pretty lake.
At Guitar Lake, you have to make a big decision for your fourth day itinerary. If you plan on camping at the summit, you'll have to carry enough water to last your group the 1.5 days from when you leave Guitar Lake, the night atop Whitney, then the five miles the next day down to Trail Camp. In my case, since I was by myself when I left Guitar Lake, I was carrying 7 quarts (15+ pounds) of water. The reward is an awesome sunset and sunrise, and knowing that you slept atop the highest place in the lower 48 states. If you sleep outside the cabin, like I did, just be prepared for a cool, windy night.
If on the otherhand, you decide not to spend the night on the summit, you don't need to carry as much water. Then when you reach the junction of the JMT and MMWT, all you need to do is leave your pack, and take only a small daypack with some water, snacks and a jacket to the summit. Then when you return to the junction, it's down to Trail Camp for the night. If you camp there, it can be a bit crowded, but it never bothered me too much.
No matter which way you go (summit or Trail Camp on the fourth night), you'll be rewarded with some cold beers and the awesome Mooseburger with fries when you reach the Portal store at the end of your adventure.
Good luck and enjoy your trip. I sure did, and plan on doing it again soon.
Whitney Mike