An 18 piece dinnerware set is odd, because a service for four usually comes in a 16 piece dinnerware set, which is the most common selection of casual dinnerware sets.
The difference is that in an 18 piece set, it is usually a service for six that includes three pieces (instead of four) in a place setting.
As a standard, a 16 piece set includes a service for four with four items in a place setting. This items usually include a dinner plate, a salad plate, a bowl, and a mug.
In 18 piece sets, the place setting most commonly include a dinner plate, a salad plate and either a bowl or a mug, or a dinner plate, a bowl, and a mug, minus the salad plate.
If you face a odd choice like this, be sure that you get the pieces that you will use the most, or make sure that extra pieces are available to purchased separately.
The salad plate is the less used in my home, so it makes perfect sense to me to purchase a service for six without that piece. However, bowls and mugs are most commonly used by my family, so I would be hesitant in purchasing a set without them. I would make sure that the needed pieces were available for purchase outside of the set.
There aren't many dinnerware manufacturers and distributors that sell dinnerware in 18 piece settings; however, Lenox and Corelle dinnerware are popular brands who are breaking the trend and providing 18-piece sets.
Here are some of the sets that are available for your choosing :
To me, it makes sense only to purchase the dinnerware pieces that you need and will use the most. Another choice is look for open stock dinnerware. That is where the pieces of a dinnerware set are sold individually. This is most common in fine china, but occasionally you will find casual dinnerware that is available in open stock.