Documentation for the SD 4 bit protocol .

Hello guys . i am working on a project on an FPGA and i want to use the SD 4 bit protocol instead of SD SPI because that seems to be more efficient and faster . the thing is , i can’t find it anywhere .
i found plenty of specs for the SPI protocol but none well written for the SD 4 bit .
some even say that it’s licensed and that i “need to get signed with 10 first born sons” before getting to it . it sound ridiculous though i am just a hobbyist and i don’t think something like that can be kept from the internet .
can anyone please address me to a valid link ?

( And before Sir Robin asks me to google this , i have been searching for three days now without some good luck )

amine2:
( And before Sir Robin asks me to google this , i have been searching for three days now without some good luck )

Sir Robin is a wise person. As proof, I found this from the first link in my search (~10 seconds of my time)...

After brief look into the linked document I think it does not have enough information to implement the protocol. You may try this document but I don’t know if it includes everything you need.

SD card specifications.pdf (1.12 MB)