Welcome to the Communiry - I will always err on the side of caution and to run a 5.1 host in your cluster you will have to upgrafe your vCenter server to 5.1 - so being a production environment I would just go ahead and add another ESXi 4.1 host and stay in an environment your are familiar with - for testing if you do not have spare harde is create a virtualized vSphere 5.1 environment either on top of VMware Player or VMware Workstation -
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