Is it possible for me to turn a crawl space into a 4th level (basement) under a 3 level split home. If so what is the easiest way to do this. Is this a do it yourself project. If not about how much would it cost to have this done?
Of the 3 ways to add to a home (up, our or down) digging out a crawl space is by far the most expensive and the least likely to add value to the property. The exact cost will depends on the size, conditions and thousands of details. A "guestimate" on my part without a great deal of additional information will not be of any value.
The fact that your home is a split, should not be a major factor here.
This is a complex job which will require engineering, detailed plans, specifications, specialized equipment and labor.