Sure, you can certainly get a teaching job for 20'000 RMB, but those opportunities are extremely rare.
What you have to consider when making a choice between a teaching job and a white-collar job is what your plans for the future are. The more you stay in the teaching industry, the more difficult it is to go out of it.
Think about the situation if you go back to your home country; does a teaching job like you do in China bring you a good salary, can the teaching experiences you get in China give you a better profile for your future employer. I cannot tell for the US or the whole world, but in Europe, I can assure you that no companies will consider your teaching experiences in China as valuable experiences, as opposed to working in a company (local or multinational).
I am sure that people working here as a teacher would agree with me. Some became teacher in China because of the lack of opportunities in their home country or here in China, some other to get more experience in teaching, but to be able to increase their salary for their next teaching job.

Yes, absolutely. However, you must, at the very minimum, have an advanced degree. Undergraduate degrees do not count for much in China anymore. You will also, of course, need at least a couple years of teaching experience. You do not necessarily need to be a professionally certified teacher, but it helps. And, most importantly, try to think outside the box when looking for a teaching position. Education is a business;  you have to go where the money is. It's certainly not in eChinacities Jobs.

Another thing to think about in regards to teaching, that 20K (or, more likely, 8K) per month is for about 12-18 hours a week work. Your 4K in business will be for about 40 hours. Many teachers do stuff on the side, and with that (if you're good, or at least good at marketing yourself) can easily double your income. And, for only a few hours more!

I'm actually going to take on a part time teaching job simply because it pays more than my current job (where I work 60 or so hours a week).  I agree that teaching in China has no resume/cv benefits other than getting other teaching jobs in China, but here's my plan:

Teach for 20 hours a week, get a decent salary, and use the huge amount of free time available to do my own side business.  If it takes off, then I can stop teaching.  If not, I can always look for other opportunities while having a job that pays ok.

Don't do what I did and get stuck in a white collar job that tries to work you to death unless you ABSOLUTELY need the experience for the future.

The thing is though, short term vs. long term.  In the short term teaching is OK, especially if you're right out of university.  But like Tibbles said it does absolutely nothing for your resume and within China actually counts AGAINST you.

From what I've heard, 5-6 years ago it was really easy to get out of teaching if you wanted to, but since the economic downturn there is that critical mass of foreign professionals here that no company would hire a teacher to do a real job.  Chinese are very hierarchy oriented, they see teachers as the migrant workers of the expat community.  

The problem with teaching is that it qualifies you for nothing, at no point no matter how long you do it will you become qualified for the really good positions if you weren't already.   Theres no advancement possible within the industry, and at this point it's damn near impossible for someone who came here out of college to move on into something else.

If you can scrap by for a year doing a white collar job even for commission only, DO IT.  To hell with those teachers who laugh at you for being poor.  If you're 25 now, in 5 years you can be making 30-40K in a management position and dating one of those euro-models, while those guys are still whining about how their schools don't respect them.

If you want to go into ESL teaching, I'd reccommend you go in with a clear savings objective, and an exit plan.

In my case, I'm working here to fund (hopeful future) language study and a masters program starting at the end of next year. Doing the same thing year after year would really not be fun.

Once you secure the right teaching job that enables you to broaden your horizons, and I don't mean working weekend and nights here, cause that certainly limits your opportunities, you should then focus on how to enhance your position by looking at the following ideas.
1. Do you have previous work experience/teaching experience? If so, and you are still somewhat passionate about your abilities, seek out ways to parlay that into other positions and consequently, more income. My background in public speaking and enabled me to branch out on a private lecture circuit whereby I visit universities and talk about English and Culture.
2. Don't limit yourself to the classroom while teaching. The biggest source of my income comes from companies and executives that want either English training or topical classes such as sales, marketing and business management.
3. The philosophy of you are what you eat applies here too. So if you are wanting a change and you find yourself frequently hanging out with buddies at holes in the wall getting shitfaced  and expecting something to happen for you...well, you know what is going to happen right? Don't get me wrong, a few beers at those little restos can be relaxing and a great way to vent at times. I'm not unfamiliar with that at all. However, after a couple of years here I learnt how to play ball so to speak. Now I have made friends that put me in the right social circles to meet the guys that are either in a position to help me further my goals here or in some cases I can help them too. You never know where that next big opportunity is going to come from. My biggest client to date was introduced to me in a resto while having beer with my wife's father. If they like you and you don't screw it up, you are on your way.
4. If you want to branch out, get a Chinese assistant. This is a role my wife handles with passion. Before her, one of my former students filled the role.
Finally I have to address the poster who said there is no possibility for advancement. That's wrong and I wish guys would not make false statements like that without proof. I have advanced through the ranks since my arrival here and am very proud of what I have done. I guess closed minded guys see no room for advancement and I suggest the guy who wrote it was either closed minded or simply misinformed.