Charles (Turnip) Townshend's invention, four field crop rotation?

I got really confused with this. Since it is a four field crop rotation thing with not much information from ANY source, I need help figuring this out. Are all the crops (turnips, barley, clover, wheat) grown at the SAME time at different ares of the field and then they rotate every year? Or is ONE crop grown for one year, then it is cleared out and then replaced with another crop to be grown for one year so it takes 4 years to grow all 4 of the crops? Please help!!