There were four rivers out of Eden that watered the whole world showing the reach of God into all existence. The compass points of North, South, East, West, are found in the natural order of creation. Two number four s signify the two creations. One is passing away in Adam while a new one emerges in Christ. Eight therefore reveals the same essential reality that two does.
Eternal life is in two four s or two creations one natural and one spiritual.
To illuminate this principle, remember that numbers have spiritual significance and each one has a voice or interpretation. Eight is the number of new beginnings, also signifying resurrection power. After a full seven days, the eighth day is the new beginning of the first day and a new seven. Jesus rose from the dead on the first day of the week, but it was the eighth day from His entrance into Jerusalem thus the connection to eternal power and the number eight. The day of resurrection is the eighth day or new beginning.