Water – Why does the water pressure drop after using the water hose for a few minutes

When I'm using my water hose, especially with the electric pressure washer, after about 15 minutes of use, the water pressure drops. I can disconnect the water hose from the pressure washer and although the hose is running full, it has a very weak stream. This isn't a problem in the house and we never notice it, even when the washer is running or if we are taking showers. I have no water leaks anywhere and this problem is present even if I'm just using the water hose without the pressure washer. My water system is residential. I have a 4" well with a submerged pump and a pressure tank in the garage. The system is four years old.

Would this be a pressure switch problem or a pump problem?

UPDATE

Thanks, guys for all the helpful answers. My pressure tank is a 40 gallon and I have not problem running out of water or the pump keeping up. I have ran four hoses at the same time watering apple trees and landscaping almost all day as well as our in house use without any problems.

Actually, I have since found the problem. I should have read the instructions to the pressure washer more carefully as it specified not to use more than a 25ft. hose connecting it to the water supply. I just connected to my hose supply which is on a hose real and contains several hundred feet of water hose. Evidently, by the time the water circulated through all those hoses on the real, it was being restricted somewhere along the line. I disonnected one hose and the thing worked like it should. I do need to up the low pressure setting on my pressure switch as I like to have a kick on pressure of 40psi and kick off pressure of 60psi and right now it is a few psi below 40 but other than that things are like they should be.

Thanks again for all your help.