Hagar is said to represent the covenant from Mount Sinai, which would mean that she represents the Mosaic covenant (Exodus 19:1–25). Sarah would then represent the Abrahamic covenant (Isaiah 51:1–2). This is confirmed by the fact that Paul quotes from Isaiah 54:1 in Galatians 4:27, thereby linking Sarah to Jerusalem while also affirming that the time has come for the eschatological fulfilment of the covenant made with Abraham.1
24 Now this may be interpreted allegorically: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar.