How much money should i bring to Beijing for 4 days and Hong kong for 3 days?

Do you mean how much money you may take and spend BJ and HK?
If so, it is hard to tell you how much will you take in both the two places, because it depends your own real situation. In general, in BJ, you may need to spend about 3000RMB.
For the limitation of curreny that you can take into China, it is no more than 5000US$.
In my opinion, you can take a small part of money and then store the rest of it in your bank cards and you can take a little bit more, so that you do not need worry about the money problem.

Since china still uses cash mostly i relied on my atm card while here. Since i bank with bank of america they have an agreement with china consstruction bank and i believe i was charged with only a 1% fee on withdrawls. My chase card had a higher fees like 5%