x1=v1=f1 is a stage 4 fu'ivla meaning x2=v2, based on word x3=f2 in language x4.
'stage 4 fu'ivla' is a term specific to Lojban and described in 'The Complete Lojban Language', chapter 4, section 7. The definition is similar to that of fu'ivla, but with the x3 place of fu'ivla omitted. x1 is assumed to be a word in Lojban as the concept doesn't make much sense for other languages. See also pavyfu'ivla - 'stage 1 fu'ivla', relfu'ivla - 'stage 2 fu'ivla' and cibyfu'ivla - 'stage 3 fu'ivla'