I would say that this practice would not be sustainable for two reasons. First, the agricultural reason would be that I believe it is unwise to grow the same crop in a field for four years straight in a row and it is better to have crop rotation so as not to exhaust the soil in certain nutrients. Secondly, economically it may not be sustainable because he is 'putting all his eggs in one basket' and assuming that the price of wheat will stay up there whereas it probably would be wiser to have other crops available in case of a drop in the wheat price.