I feel your pain. It seems like my son skipped most of the terrible twos but came back with a vegeance with what I call the horrible threes. He also has a speech delay and since he was 2 1/2 was in individual speech therapy, provided thru the regional center where he was evaluated. As mentioned before, part of the work up for all speech delays has to have a hearing evaluation, as diagnosing and treating hearing problems can make a great difference in their speech. My son's hearing is fine. He got a complete evaluation from the regional center and apart from the speech delay, which we obviously knew about, was deemed to be a sensory seeker and was recommended to have occupational therapy added, so he was switched to a place called Pediatric Therapy Network for the last 2 months before his third birthday, and those 8-9 weeks there made a huge difference. He learned to sit down for activities, where before I would be exhausted from trying to make him hold still during every single toddler or mommy and me class we took. I was sorry he had to "graduate" (they only take kids up to age 3), but he was referred to the school district, where he got reevaluated by a psychologist, occupational therapist, and a speech therapist, and then we had an IEP, which is a meeting where a social worker, your counselor from the regional center if you have one, a psychologist, speech therapist, and occupational therapist from the school disctrict meet with the parents, review the results of their assessments, and based on that decide what type of class environment your child should be placed in. You can also take reports from any other people who have worked with or evaaluated your child, or even have those people assist the meeting and give their professional assessment. There are 3 types of classrooms and before the IEP you can have your social worker show you the 3 different classrooms where you would be potentially assigned, since you have a say in what is decided, since after all, it is your child, though you might not always get what you want, from what I've heard. We had a very positive experience. My son has been in the preschool special ed program since he turned 3 and continued with classes during the summer, called ESY, extended school year, and it has been amazing how the last 6 months have helped his speech. He went from a few 2-3 word sentences to 4-5 word sentences and his vocabulary has increased almost exponentially. He gets additional speech and occupational therapy thru the school district during school hours, (they consider every minute he is in school IS speech therapy, since the are enforcing speech in every activity) as well as an extra hour of OT (occupational therapy) per week, to help him burn some energy and get used to following orders or doing activities he doesn't like.
So bottom line, there is a long road ahead of you, but there is light at the end of the tunnel. There are things that have to be enforced by you at home and when you go out, explain limits and consequences, and all the other things people mentioned, but also talk to your pediatrician about all these concerns; although there are not many available, and depending on your insurance too, your pediatrician might be able to send you to a behavioral specialist for an evaluation, or if you have a PPO you can find one in your plan and do it yourself, and even if you don't have a PPO, nowadays you no longer need a referral for a psychologist with most insurances, you can look up who is contracted with your plan and call them directly, the more resources that you can find and use, the better at this point. I also would recommend going to the regional center. It is true that after age 3 the school district is the one in charge of providing all the services, BUT I believe there is an After 3 Intake at the regional center, and they might either evaluate her and refer her to the school district (they set up everything with them for you) or at the very least get you connected with the school district and have them go from there. Since this will take some time, by then she'll hopefully already be in speech therapy, and you'll have one more person who knows, works, and can advocate for your daughter, and work in conjunction with her teacher and/or speech therapist at school; if her vocabulary at almost 4 years is this limited, I would definitely fight to have speech therapy at school as well included in her IEP.
It will help her in many ways to go to preschool, which is just 2 1/2 hours a day, and interact with other kids, and she can even use the school bus to be picked up or dropped off at your sitters, as long as that person is within the district boundaries, so that it won't interfere with your work.
Good luck!