Making a 4" hole in my foundation , will cutting the rebar cause the foundation to crack?
Cutting the re-bar so you can complete your 4 inch hole could cause your foundation to crack. I suspect that you already know that or else you would not be asking this question.

The problem here is that there is no way to provide any answer regarding assurance that cutting the re-bar would not be harmful to your foundation. There are just to many unknown variables to be able to make any prediction. Some of these unknowns are:


Are there other re-bars in the foundation
What part of the foundation, wall or footing 
Type of construction of the foundation, cement blocks, poured concrete, stone or bricks
How thick the foundation is
Where the hole is located relative to edges, top, bottom or corners in the foundation
Hole location above or below grade
Past history of building structure foundation
Temperate zone location
Seismic zone conditions