I just spent some time with the Apple iPad Mini 4 and the screen protectors meant for iPad Mini 3 will be not be compatible with Mini 4. This is because the iPad Mini 4 is taller so do not waste your time in buying older screen protectors.
So while on the outside, the Apple iPad Mini 4 may look same as the Mini 3 but they do not have identical dimensions. It is taller and has thicker top and bottom bezels.
The Mini 4 has these dimensions, 8 inches (203.2 mm) x 5.3 inches (134.8 mm) x 0.24 inches (6.1 mm) while the iPad Mini 2 measures 7.87 inches (200 mm) x 5.3 inches (134.7 mm) x 0.29 inch (7.5 mm).