Green manure is obtained from decomposing green legume plants. Green manure refers to plants that are grown specifically to be turned into the soil to increase fertility and improve its physical properties. Green legume plants, such as clover, alfalfa, and vetch, are commonly used for green manure.

Option 1, domestic vegetable waste, is not a source of green manure. While vegetable waste can be composted and used as organic matter in the soil, it is not specifically grown for the purpose of being turned into the soil as green manure.

Option 2, oil seed husk cakes, are not a source of green manure as they are byproducts of oil extraction and not typically used for improving soil fertility.

Option 3, fresh animal excreta, is also not a source of green manure. Fresh animal excreta can be used as fertilizer, but it needs to be properly composted before being applied to the soil.

Hence, the correct answer is 4.