What is 4mm f2 8 on aps c in full frame?

4mm on APS-C gives the view of 6mm on full frame. For the same depth of field, f/2.8 there behaves like about f/4.2 on full frame. Exposure doesn't change: f/2.8 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 7.6mm at about f/5.3 for the same depth of field
• Full frame: 6mm at about f/4.2 for the same depth of field
• APS-C (yours): 4mm at about f/2.8 for the same depth of field
• APS-C (Canon): 3.8mm at about f/2.6 for the same depth of field
• Micro Four Thirds: 3mm at about f/2.1 for the same depth of field
• 1-inch: 2.2mm at about f/1.6 for the same depth of field
• 1/1.3-inch: 1.7mm at about f/1.2 for the same depth of field
• 1/1.7-inch: 1.3mm at about f/0.9 for the same depth of field
• 1/2.3-inch: 1.1mm at about f/0.7 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.