Is there any difference in the strength of open web floor joist with 4x2 or 2x4 construction of the webs, that is the members with the long dimension being either horizontal or vertical? I understand the 4x2 orientation gives more interior space within the truss but is this at the expense of strength.
Floor trusses are made with load limits in mind. If there is a higher anticipated load than what would be considered standard, the truss design will be modified sufficiently to accommodate the additional load. Whether or not the individual members are oriented 4x2 or 2x4 is negligible. The strength of the the truss has been calculated and deemed sufficiently rigid for the application. That being said, what is it that you are trying to accomplish? With more specific details, a more specific answer can be given.