John Gill Commentary Joshua 8:4

"And he commanded them, saying, Behold, ye shall lie in ambush against the city, behind the city; go not very far from the city, but be ye all ready:" — Joshua 8:4 (ASV)

And he commanded them, saying
At the time he sent them away;

behold, you shall lie in wait against the city, [even] behind the city ;
that is, on the west side of it, (Joshua 8:9) ;

go not very far from the city, but be you all ready ;
to enter into it, as soon as the forces are drawn out eastward to meet the army of Israel.