Calculating Air Pressure Loss

4" is a good size pipe in order to have a pressure loss would to be using a lot of cfm's.

In order to calculate the loss would need to know what the cfm's are.

http://www.engineeringtoolbox.com/pressure-drop-compressed-air-pipes-d_852.html

http://www.engineeringtoolbox.com/compressed-air-pressure-drop-nomograph-d_1278.html

http://www.engineeringtoolbox.com/compressed-air-pressure-drop-fittings-d_1279.html

To do it "from scratch" there will be examples of this in any fluid mechanics text book. It of coarse involves the Reynolds number. You would need the right prerequisites to understand the material.

Many "typical application" tables or algorithms exist such as the good one referenced in response two. They work well when used as intended.