Usually, the crop conversion is a ratio of the diagonals of the two formats. If I did my math right, it is a crop factor of about 1.3. however, you are going from a square to a rectangle and I find the psychology of how we percieve the two areas is different. I would suggest dropping a 4x5 mask in your Hassleblad viewfinder and looking through it.
BTW, is 4x5 the sensor size or the area of the sensor used to capture the image? There is a small difference both for the crop calculation and mask size. But the mask should give you a feel for the new format.