What download speed is neccessary to donwload 1.2 Gigabyte of data
from a server in US to China in 4 hours.

How fast is your modem???

1.2gb in 4 hours
300mb in 1 hour
(300/60) /60 = 0.083' m bytes per second
0.083 x 9 (8 bits + 1 stop) = 0.75 m bits per second

0.75 * 1024 * 1024 = 786432 bits per second = 786432 Baud

Ignoring the stop bit - since you'll not really be using a modem :-

699051 baud

Of course that is 'effective baud'
I would guess that on a good day, with the wind behind you a 1 MBit
connection should do the trick. But theory and practise often differ.