Question
How far apart would we plant a single row windbreak of colorado blue spruce. The trees are aprox. 4ft. now.

Answer
They have a spread of about 10-20 feet meaning if planted 10 feet apart the limbs  will meet near the bottom of the tree. If you want them to grow together forming an almost solid wall I would plant about 5 -6 feet apart. Dig the holes twice the size of the rootball and as deep and fill with good top soil or potting soil and mulch around the trees with not more than 3 inches deep of organic mulch not piled up on the trunk (pine straw is good).

OR you could plant them if you have enough in two offset rows 10 feet apart This will give the hedge appearance but it will take about twice the number ot plants.