If she needs some time to warm up to a new situation, that's OK. I have a friend whose child is terribly uncomfortable in new situations and we've brainstormed together for ways to help him out. One thing is that you might turn your focus more to preparing her for new situations, and enlist her help. If you're getting ready for a playdate where new people will be there, tell her about it over breakfast. If you know the new kids' names, tell her their names and see if she can remember them (maybe a sing-song quiz while you're in the car or something). Ask her what she'd like to say to a new friend. "We're going to see a new friend name Chloe today. She is 4 too. What do you think you might say to Chloe when she's introduced to you?" If she shrugs her shoulders, ask if she'd like some ideas...then give her a few -- Maybe you could say "Hello Chloe, how are you?" or "Hi! What's your favorite color?" Whether or not you know the names, just do your best to prepare her in advance for anything new. Tell her the name of the place you're going to, decribe it to her, quiz her (in a fun way!) on different greetings she can say to new people, etc. I would hesitate to sign her up for a bunch of things without preparing her for it or you could have the soccer situation all over again. If she goes to preschool and swimming lessons, that seems like plenty for a 4-yr-old schedule. And I wouldn't worry about kindergarten, especially if she's in preschool already. She should be well prepared.
Also, have you talked with her about why she doesn't want to do things like school or swimming? Maybe something happened that embarrassed her or there's a bad relationship with one of her friends? Or maybe she had a dream related to something there that has made her uneasy (One friend's daughter, also 4 yrs old, developed this bizarre fear of teapots. For weeks she went around the house covering toy teapots with towels, freaked out when her mother got the real one out to make tea, etc. She could never figure it out until one day the girl ran into the kitchen to look at the kettle and said "It's happy now!" -- only then did she find out that the kid had been having a recurring dream about an angry teapot. So, you never know what could be going on in a 4 yr old's mind!!)
Try to engage her in a conversation about it. Ask her what makes her sad about going to school and swimming. It could be as simple as a renewed sense of separation anxiety.
Good luck!