Can I encrypt my SD card using the HSM4? (I know I can do this with the Zymbit).
Yes HSM4 can encrypt you SD card using the same code you have developed on ZYMKEY4.
HSM4 has all the same functionality as ZYMKEY4, just in a different physical package, soft binding lock and external battery.
FYI, a full feature comparison here. Compare Modules
Would you say the HSM4 is more secure than the Zymbit? (I have both). I am asking because the HSM4 seems to be enclosed in a case - unsure if its tamper proof.
As a component the HSM4 is inherently more secure because it’s chips are not exposed. That said the HSM4 is really designed to deliver essential security to the system (RPI, SDCard, HSM, Enclosure), so to get the benefits of ‘more security’ you still need to ensure you use the tamper circuits to protect your entire system assembly (RPi, SDCard, HSM, Battery).