You are unable to take a snapshot in ESX 4 and 4.1 with Protect 9.1
Support for ESX 4 and 4.1 was removed in Protect 9.1 due to VMware is ending support for these platforms in 2014. Shavlik Protect 9.0 is the last version to support ESX 4 and 4.1.
Scanning ESXi 4.1 Hypervisors are still supported in Protect 9.1
Additional information can be found in the Release Notes Shavlik Protect Standard/Advanced Release Notes for 9.1