Leyland cypress chips are better at being a mulch than a soil conditioner, unless you have really small chips. They will take at least a year (more likely 2 or 3 years) to decompose. Keep in mind that 4 inches of chips will eventually decompose down to less than 1/2" of improved soil... if none of it is washed away. The weed we usually call buttercup often signifies very poor soil, so anything that will improve the soil is better than nothing. If you were considering turning the area into a garden, you would probably be better off doing "lasagna" to improve the soil and using the chips solely as a top-mulch.