If it incourages you there seems to more than one of them alters for all 4 boss's, I found 3 Elders register stones and 3 Elder alters. Same with Bonemass 2 of them.
Also 2 of the others.
For Modor I found his register inside of one of the towers in the mountain. His first alter I that was shown on the map is also in the highest mountain.
One of these stone towers
https://steamcommunity.com/sharedfiles/filedetails/?id=2431129597