Measure Sourdough Starter; let stand at room temperature for 30 minutes.
In a mixer bowl, combine 2½ cups of flour and yeast. Set aside. In a saucepan, heat and stir water, sugar, butter, and salt just until warm and butter almost melts. Add water mixture to yeast mixture, then add Sourdough Starter.
Beat on low to medium speed for 30 seconds. Then beat on high speed for 3 minutes. Combine remaining 2½ cups of flour and baking soda; add to yeast mixture. Mix with a dough hook on low speed until combined. Add in as much of the remaining 1½ cups flour as you can.
Turn dough onto a lightly floured surface and knead 6-8 minutes. Shape dough into a ball and place in a lightly greased bowl, turning once to grease surface. Cover and let rise until double (about 45 minutes). Punch dough down and divide in half. Cover and let rest for 10 minutes. Coat baking sheet with cooking spray.
Shape each dough half by gently pulling into a ball, tucking edges underneath. Place on baking sheet. Slightly flatten each dough ball until about 6 inches in diameter. Make crisscross slashes across loaf tops. Cover and let rise for 30 minutes. Preheat oven to 375 degrees. Bake for 30-35 minutes or until bread sounds hollow when lightly tapped. (If necessary, cover loosely with foil the last 10 minutes of baking to prevent over-browning). Cool on wire racks.