All depends on how long before the gelding, and also on his temperment. I would assume he'd have to be gelded some time before being shipped, simply because you wouldn't want a horse recovering from the surgery while shipping across the continent.
Due to him being shipped, I would be more inclined to keep him seperate for a little while anyway, just to make sure he adjusts before stressing him out more. If he's been a stallion for 4 years, he may need some time to adjust to living in a herd, so wait until he is acclimated to the totally new environment then start putting him in with other horses.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."