baudrate is computed with the following formula :
baudrate = (2^smod)/32 x (osc/(12x(256-TH1))
with smod=1 and osc=10.10^6 and TH1=245d

Baudrate = 4734 Bits/s which is 4800 bits/s with 1.37% accuracy.

I've already done some serial communication with this accuracy on the baudrate but with 1200 bits/s baudrate and a 20MHZ crystal.

To be sure that the accuracy isn't your problem, you could check by trying with an other crystal frequency (11.059 and 18.432 are giving some very precisely baudrates)


Stephane