Hi Rajendra
Kindly follow the steps & could you share your OS / DB size details
1. System copy from live Production to test system copy, in test system you are going to start the upgrade.
2. Do the upgrade ECC EHP 4 to 6 in test system, during this process you have to calculate over all timings of upgrade process of SUM and all other issues.
3 After upgraded to ECC6 EHP6 in the test system, do the full function level test with help of ABAPer /Function consultant.
4. Based on the test environment result, you have to perform the same upgrade process in live production system with minimum down time, during this process users must be locked and all other background jobs are suspended.
Regards
Sriram