There is no need to get all 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 four season tents and regular tents is that a four 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, four 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 designed, the more specialized a 4 season tent will be. For example, some true mountaineering 4 season tents are not things that you would like to take camping on a warm summer night.
The ventilation panels are small, with the result that it will be stuffy and hot at night if you are camping during the summer with them. Additionally, they are made of heavier, stronger material, and as a result are an added burden for your normal camping trip.
And of course, good 4 season tents can be prohibitively expensive, running upwards of $500 dollars sometimes. Compare this to a 1-2 person summer tent that you might buy at your camping store.
I got mine for $30 dollars and, although it provides little warmth, it works alright during the summer when combined with a decent sleeping bag.
The truth of the matter is, it is important to think about what you will be doing before you go out and buy expensive mountaineering equipment and 4 season tents that you will not need. You must make sure that your equipment fits its purpose.