There is no need to get 4 season tents for gentle summer camping. Even if there is a heavy downpour, the use of sleeping pads combined with adequate planning should keep everything fairly dry. The important thing is to not camp at the bottom of a hill, to make sure the rain fly is secure, and to take advantage of natural cover.
But, a four season tent can be a nice thing to have for extreme conditions, and if you have the money to spend on one, it is a luxury that can really improve your camping experience.
Basically, the difference between 4 season tents and regular tents is that a 4 season tent is tighter, with heavier outer walls. When it is all zipped up, there is no space anywhere for the elements to get in.
In addition, 4 season tents are often stabler so that they can resist extremely heavy storms if need be. This does not mean, however, that you cannot use 4 season tents in nicer weather. Many styles of 4 season tents come with the ability to unzip the outer fly so that you can keep cool on summer days, while still braving near-arctic temperatures in the winter.
You will find, however, that the more extreme the conditions for which it was...