Is it bad to have had 4 jobs by age 18?

As long as you aren't commiting any crimes or have some anger problem nothing is bad at age 18. It is good that you have worked.

Think of it in more of a positive light. You're only 18, so nobody is going to expect you to have a lengthy resume where you've been employed for 7 years at one job for example. If I were an employer, it would look to me like determination. The job market is hard out there, and seasonal jobs are not the nicest. Regardless of those things, you had jobs and you are out there looking, which is more than I can say for a lot of people your age. I say good on you. At least you're up there having a go!

No, but I'd not on a resume or application the two that were seasonal.

Relax sweety. No it doesn't look bad because they were seasonal, and it's retail jobs while you're a teen. Trust me, nobody cares. No one at all. You won't even be using these jobs as references when you enter the adult world -- assuming you go to college or something and start getting real jobs.