There is an Instrument Driver for the 4194A available for download at the following site:
http://zone.ni.com/idnet97.nsf/9b2b33e1993d877786256436006ec498/c09ed54b33083835862568ab005fbaa4?OpenDocument
You might already be using it. If there is some functionality you would like to have added, you can submit a request at:
http://zone.ni.com/idnet97.nsf/instrumentdriverrequest/