I completely believe that the 4-wheel suitcase is an under appreciated object. While I used to see passengers walking around airports with 4-wheelers and think that the people must be very strange, my opinion has completely changed after owning a set. Rolling on four wheels makes taking heavy bags from place to place much lighter and easier on your arms. One thing that I have found particularly useful is the fact that if one wheel happens to pop off, you can turn the suitcase onto one of two sides to roll it on two wheels, and continue on your trip without having to drag your bag (as you would have to do if you only had two wheels originally).
The outside of my suitcases have a nice feature, too. The plastic is hard enough to protect your possessions from being hit on the inside, and does not tear as fabric suitcases do. I also was able to buy these in a set, with the smaller one fitting inside the larger, great for storage. I also can use the small one for week-long trips, which is super convenient.
The one drawback I have with these suitcases is with the larger suitcase. It is extremely easy to go over 50 lbs. and pay a fee at the airport. Without even zipping open the expansion area, you can fill this bag up without realizing it is way over weight, only to get to the airport (if you forget to weigh beforehand), trying to shove all your stuff into your carry-ons.