Hi Amit,
Actually, bit rate of each of the channels does not change. If a channel is sending 10Gbps, then at the receiver side, each of the ONUs receive with a bit rate of 10Gbps. What splitter does is splitting the power (and not bit rate). Then each UNO receives their own wavelength and the total bit rate is 10Gbps x 4 = 40 Gbps.
Regards