Yes, according to the provided information stabilization for the main regulator of part U4 is achieved by connecting an external capacitor C_EXT to the VCAP1/VCAP2 pins. Specifically, the capacitance of the external capacitor should be 2.2 µF and its equivalent series resistance (ESR) should be less than 2 Ohms[2]. It is important to note that when bypassing the voltage regulator, these two 2.2 µF capacitors are not required and should be replaced by two 100 nF decoupling capacitors[2].

[2]: Page 95

According to the information from the document U4, the STM32F469xx devices can operate within a supply voltage range of 1.7V to 3.6V[1]. Therefore, you should be able to power U4 with 1.7V. However, be aware that functioning at the extreme end of the power supply range may affect specific features or performance of the device. If you plan to use features such as USB full speed PHYs which require a voltage range down to 2.7V when $V_{D D U S B} $ is connected to $V_{D D} $ [2], those may not work as expected with a 1.7V supply.

[1]: Page 12
[2]: Page 95