I think you hit the nail on the head in including all the possible causes of your son's dramatic behavior change.
Yes, there's a new person emerging around four. I was a bit thrown for a loop with my own son this year (he's 4.5 too). The changes you mention are all things he has no control over, so yes, he may be trying to flex his 'autonomy' muscles to find out what he does have control over.
The book that I like (and one that my son's preschool recommends as required reading for parents) and have used for years is JoAnne Nordling's "Taking Charge: Caring Discipline that Works at Home and At School". The first focus of this book is about ensuring our children feel safe, secure, and their needs are being met. Then, she identifies four categories of misbehaviors and provides corrections for them that, once practiced consistently, are relatively simple to carry out. Her biggest attribute, in my opinion, is her focus on bolstering the child's self-worth with lots of positive attention during times when the child is showing 'neutral' behavior (not positive or negative, just doing things *they* enjoy doing). Her focus on honing in and drawing out a child's internal reality is also a revelation: what we think is going on with them, their perceptions, is sometimes not what we think at all.
I found that when my son had some types of outbursts, I could ignore them as self-indulgent behaviors. Some times, I could just repeat a short phrase until he did what was asked (the technique is called 'the broken record'). I save time-out only for dangerous/aggressive behaviors, and that helps a lot. My son was so similar to what you described, a very docile two year old. We haven't really even had tantrums until this year, so it was quite an eye-opener. Good luck and like I said, this is a book I highly recommend. (You can read reviews and find it on Amazon.)
One other thing: at this age, we sometimes want to start negotiating or asking 'if we do X (a few more minutes of tv, etc), will you be cooperative?'. Don't Do This. This is not politeness, this is a trap. Do not ask permission from your child to do what you need to do. You are the adult and know what's going on. The few times I've tried negotiating with my son, I regretted it right away. He was happy for a few minutes and then was a stinker about what needed to be happening all over again. Be respectful with the transitions you describe, but when it's time to turn off the tv or leave, do what you need to do. It's an age-old line of parenting we are a part of. Be the mom.:)