‘ And so it was that, when the people were clean crossed over, the Ark of YHWH crossed over, and the priests in front of (in the presence of, in front of the eyes of) the people.'
Once all were over and the stones in the Jordan were in place, and the other stones were on the bank waiting to be set in their night's lodging-place, then the priest's finished their crossing, bearing the Ark of YHWH ready for the battle to come (see Joshua 4:11-12), in front of all the people.
“In front of”. ‘In the presence of the people' or ‘before the people', compare the use of ‘before' in Joshua 3:5). It means in front of their very eyes.