S., I've coached 3-4-5 year olds in soccer for the past two years.
My advice is let her quit. If she is matter-of-factly telling you she doesn't want to play, don't force her. There will be resentment and it will make the coach's job that much harder if she is forced.
Some almost-4's are ready for the social and physical demands of team sports... others are not. I'm not a big fan of pushing any age kid in sports (although I strongly encourage those "on the fence" to try it). If you've gone through two practices with the same result in your daughter's attitude, please pull her out and maybe try again next year.
Also, there's nothing wrong with going to the games and watching & cheering! Maybe you have a cheerleader/dancer on your hands :)
Fyi - my oldest (now 6) started soccer when she was 4 1/2 and although she loves the game she is not an aggressive player. My middle is now almost 4 and has played for 2 seasons - she did cry during practices the first season but she herself insisted she stay on the team because she wanted to play in the games. So, I know what you are going through!
Explain to your daughter that you understand she feels scared and that it's okay not to play. Suggest watching games and let her pick a team to cheer. When the fear of actually playing has passed, let her know that if she wants to try playing again, it will be okay to do so next season (or don't bring it up until next registration time, then ask nonchalantly if she'd like to try again).