what fraction of a medieval army can be described as well trained ? the bolton have 4k men and they seems very good at all….

Wildly varies by time and place. 

If we’re talking the Saxon army at Hastings, there was a huge difference between the men of the fyrd, who were farmers called up in times of emergency, and the huscarls, who were professional soldiers. 

On the other hand, if we’re talking about the mercenary companies of the 14th century, those were entirely professional forces.