4 year old used in cart too much?

There is a black mare cob called gwen, she's 14hh and is only 4 years old. My friend is buying her in september as that is when she will have the money to do so. But, her owners now, use her in the cart 5 times a week, they take her out for 5/6 hours on week days and on weekends, it's about 7/8 hours, the horse isn't under nourished but she has lost a bit of weight. Is this too much work for a 4 year old?