I'm the one in charge of the Collaboration forum, so I guess I should have a bit of insight to share on this.
1. I can do some procedure like BFS (Breath First Search), but I can't do is probably online system. (Well, I think given some time I can understand the system) so 4$/Hour considered as expensive or not?
$4 an hour is
way below minimum wage, so keep that in mind. Just to give some examples, if someone's just looking for a cheap way to get a job done, they might find that offer enticing enough. Someone with a professional background might be alerted by it and jump to conclusions about your skills or reliability, though. To appeal to the latter category, however, you'll have to have built a reputation for yourself already, as there are plenty of fish in the sea that are already established and will likely be favored.
To start building your reputation, I suggest starting lower than average, but not
too low. It wouldn't be fair for yourself, and
too cheap offers tend to make you look less qualified than you actually are. It's important to find the sweet spot between "greedy" and "baby's first freelance job", as neither of these are impressions you want your potential clients to get. Ideally, that would be "affordable newcomer indie dev".
2. Usually people came at me giving offers then just gone, no follow through. Is this normal, or I do something bad that turns off the offers?
That's pretty normal. Chances are they already found someone else to work with and took their offer, or they were just testing the waters to see if you're suitable for their needs and may or may not get back to you at a later point in time, once they have a better idea of their budget and can start gathering contractors for development.
3. Is okay to ask and see the game design document first, then giving the rates before signing disclosure?
I wouldn't ask for a complete design document up front. It tends to make people nervous.
The reason why you're supposed to sign NDAs before seeing any material is specifically so you won't reveal any of the things you're
about to see. An NDA is not a legally binding contract to work for anyone - it just means you can't reveal the related information. Asking for information before that is therefore contradictory, and you will likely be turned down if you press people not to put you under an NDA in fear of you stealing their ideas.
NDA first, if required, then ask for a generous description of both the job and the project. Negotiate rates and, if both parties are happy with them, a contract related to the actual job can then be made. If either of you doesn't like it, part ways and never talk to anyone about anything that just happened. No harm done.