John Gill Commentary Numbers 4:19

"but thus do unto them, that they may live, and not die, when they approach unto the most holy things: Aaron and his sons shall go in, and appoint them every one to his service and to his burden;" — Numbers 4:19 (ASV)

But thus do unto them
As after directed:

that they may live and not die ;
live long, and not die a sudden and violent death:

when they approach unto the most holy things ;
the ark, the shewbread table to take them up and carry them:

Aaron and his sons shall go in ;
into the holy and most holy places:

and appoint them in everyone to his service, and to his burden ;
what everyone shall do, and how they shall take them up, and bear them.