Hi Anshu,
For one SD-4 has lesser capacity than SD-8 as SD-8 devides into 8 sub slots per SD-Multiframe and SD-4 into only 4 sub slots.
SD-4 are usually configured when we are using the BCCH in combined configuration where SD slots are incoorporated in the BCCH TS. SD-8 is when seperate SD-TS is defined.