I think the reason FMatrix wants four FVector3’s is because you use them to build a matrix transform. IE: first column is vector i of the transform, second column j, third k, and then the fourth column would be the translation. Since FVector3’s got fed into the columns, the 4x4 matrice’s last row will be blank, because it’s not needed for representing a transformation.
Take that with a grain of salt though. Haven’t tested it yet myself.