Brian,
I have done literally hundreds of rehabs over the last 8 years and many of them had foundation problems. With the info you have given, there is no way to accurately guestimate the costs.
That being said, if the house is in Tarrant County, TX, it is a single-story home that is less than 2,000 SqFt., AND it is actually the foundation (as opposed to a slab leak), it is rare Straightline Foundation cannot fix it for less than $15,000 (usually much less).
One thing I would suggest you factor into your numbers is that lifting a house 4" usually involves breaking pipes. Repairing those can add a lot of extra cost FAST (i.e. $1,500 - 2,500+ / per break).
If there is anything else I can help you with, please don't hesitate to let me know.
-Greg Wilson ("The REI Mentor" - www.reimentor.com)