What is 17mm f4 on medium format in full frame?

17mm on medium format gives the view of 13mm on full frame. For the same depth of field, f/4 there behaves like about f/3.2 on full frame. Exposure doesn't change: f/4 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33) (yours): 17mm at about f/4 for the same depth of field
• Full frame: 13mm at about f/3.2 for the same depth of field
• APS-C: 9mm at about f/2.1 for the same depth of field
• APS-C (Canon): 8.4mm at about f/2 for the same depth of field
• Micro Four Thirds: 6.7mm at about f/1.6 for the same depth of field
• 1-inch: 5mm at about f/1.2 for the same depth of field
• 1/1.3-inch: 3.7mm at about f/0.9 for the same depth of field
• 1/1.7-inch: 2.9mm at about f/0.7 for the same depth of field
• 1/2.3-inch: 2.4mm at about f/0.6 for the same depth of field

Crop factors are by the sensor diagonal, rounded as they're usually quoted; a format with a different shape (4:3 against 3:2) frames a little differently across the width. More in the focal length converter.

Computed from the sensor sizes, not written by a model.