Government agricultural officials have confirmed another four cases of foot-and-mouth today, raising the present total to 24 and making the present outbreak bigger than that of 1967.   The six confirmed cases were discovered at two farms in Powys, Wales; a farm at Llangowan, Herefordshire; a sheep farm at Stonesby, near Melton Mowbray, Leicestershire; a farm at Highampton in Devon and at an abattoir at Great Harwood, near Preston, Lancashire. Officials are also investigating the first suspected outbreak of the disease in Northern Ireland and a case in Lincolnshire.