12 bytes/8 bytes/16 bytes/4 bytes

There's no obvious way to get 32 bytes, which is the typical length of a private key. I guess it could be that the last 4 bytes are not anything and the first 3 chunks are 32 bytes + 4 byte checksum. If it were and you had those question marks in those places, that's 10 question marks or 40 bits that you have to roll through or 2^40 of attempts. That's about a trillion different possibilities, which is doable, especially with some GPUs. I did 2^44 in about 2 days a while back and that cost about $800 to rent the requisite GPUs if I remember correctly. So, 2^40 should be 1/16th of that, or 3 hours and $50? But there's some custom code that needs to be written to calculate the address and there's no guarantee that that's what these letters mean.