The data sheet for the IMX521 sensor already appeared on 
in July 2019 on the Internet, but only now a user in the EOSHD forum once again pointed out what a potent sensor is offered here by Sony. And with it also our attention for this unusual sensor won.
The IMX521 is basically a magnification of the IMX294, which is also used in the Blackmagic Pocket Cinema Camera 4K and the Panasonic GH5s. It has a special Quad-Bayer structure that virtually divides each sensor into four subsensors.
In an HDR mode, two subcells of the same color can be read with shorter exposure times. With both the GH5s and the Blackmagic Pocket Cinema Camera 4K, the Sony sensor achieves a very good dynamic range for MFT conditions with this trick. But instead of an MFT sensor edge length of approx. 4.63 µm, the sensors of the new IMX521 have remarkably long 7.52 µm edges. The sensor surface is thus more than doubled compared to the IMX 294, which should result in quite impressive dynamics for a 4K full frame sensor. In addition, the active horizontal number of sensors is about 4,800 pixels, which is a pragmatic surplus for downsampling to 4K-UHD or C4K.
The probably very high light sensitivity as well as the rather low resolution for FullFrame suggests that this is the sensor of a Sony A7SII successor. But since the sensor is also available for third-party manufacturers, it could also be used in upcoming 4K FullFrame cameras from Panasonic and Blackmagic.
4Kp60 are possible with this sensor as well as smaller and slightly cropped resolutions that can be read out with 100fps and more (e.g. 4KUHDp100 with a 1.3 Crop). Interesting is also the sentence: "...this sensor also has an electronic shutter function with variable storage time". Because if the readout values of individual sensors can still be stored for a short time, rolling shutter problems can theoretically be avoided.
In short: With this sensor, the 4K dynamics should be improved by at least one f-stop compared to current 4K full frame top models such as the Panasonic S1H, and perhaps this will also lead to a significantly improved rolling shutter behavior. Coupled with 10-bit log or RAW recording, this should make it possible to announce exciting new cameras for next year...
