Paul closes chapter 4 with a complex analogy drawn from the Old Testament.  He is contrasting the Judaizers with the true Christians.  The representatives of each side are:

​

           Ishmael -- Hagar -- Mt Sinai  -- Jerusalem  --         Old Covenant   -- born of flesh
           Isaac    -- Sarah --  _______ --  Jerusalem above -- New Covenant -- born of Spirit  --  children of promise  

 

           This is a bit messy, because the pairs are unequal.  Nevertheless, his meaning is plain, and bold: 

​

           "the present city of Jerusalem... is in slavery with her children (Gal 4:25)." 

​

Those who still follow the Old Covenant, and this includes his opponents, the Judaizers, will not inherit the Kingdom that belongs to the sons of Abraham.  In a sense, he is turning the weapon of the Judaizers upon themselves.  "You want the Old Testament?  Here, I give you the heart of it --  and by it you are condemned!"