I followed build instructions and I was able to build the recovery image 
[100% 8355/8355] build out/target/product/FP4/recovery.img
#### build completed successfully (43:03 (mm:ss)) ####
Image is booting correctly in TWRP 3.6.0_11-0 
however the file differs from yours:
SHA256 hash di recovery.img:
7f9bc1836f83de679eba2992176ce8d25b1190e0a666d8a3f19e1c689efaf6c9
SHA256 hash di .\twrp-3.6.0_FP4-UNOFFICIAL-20211216.img:
9358dc6e623d213992e1c1c14b340f07524641491453f3c7065b723e4024df46
Is accessing encrypted data a general-TWRP function that is still a work-in-progress, or is this somehow already available and needs to be ported specifically to FP4?
Having this function will help a lot making backups and resolving some stupid mistakes 