Hi Sudhakar,
4 hours of business downtime during upgrade should be an acceptable downtime unless the customer needs to reduce to down to < 1 hr.
In such a case NZDT solution from SAP should be useful. I share below some docu links for NZDT solution from SAP for your understanding.
Near-Zero Downtime Maintenance for SAP Business... | SCN
Hope this helps.
Regards,
Deepak Kori