I'd never heard that one, Katek so I googled it. I think it must be a "fourth part" of a stone.
This was on https://uk.answers.yahoo.com/question/index?qid=20070911042158AAqNxBm (from David C)
"Best Answer: I haven't had a fourpit o' tatties since I was a kid visiting my Aunt in Glasgow some 30 years ago. Do they still sell them from the vans? There was a little mobile vegetable shop from the co-op, and I think it was about 3 1/2lbs weight of potatoes and they had to be Kerr's Pinks as well.
I think it was related to the old Scots measure - firlot - a dry measure equal to 4 pecks, and the boll was equal to 4 firlots.
The peck was equal to 4 lippies or 'forpets'.Since all grain and seed were measured out in Peck buckets, the small forpet measure was ideal for carrying potatoes, it was a volume measure rather than a weight measure.
Scottish for a 1/4 part, generally meaning 1/4 peck; thus the forpet is another name for a lippie. The original old Scots name for this - lippie was about 2.27 liters for wheat, peas, or beans and about 3.04 liters for barley or oats, and were made from steam formed leather and wooden buckets, with rope handles. "