Technically yes, but I would not recommend it.
The 4.1 IEV can monitor a 5.1 Sensor, but you could see some issues.
Not all possible IEV options for querying the sensor will work, and new fields in 5.x will not show up in the viewer.
The recommendation would be to change from 4.1 IEV to 5.2 IEV which is fully compatible with 5.1 sensors.
The 5.2 IEV is a no additional cost upgrade covered under your sensor's support contract.
5.2 IEV can be downloaded from here:
http://www.cisco.com/cgi-bin/tablebuild.pl/ips-ev