Just a 404 error? Looks like it could be that the server migration is not done yet. When a host move your website to another server, you don't have to reinstall everything, that's a standard procedure that they do and most of the times it doesn't affect your site. That was a warning just in case something goes wrong.
You didn't have to reinstall everything. Since you did, I have few questions:
Did you just exported or save a backup of the entire database?
Did you do all of these: http://docs.elgg.org/wiki/Duplicate_Installation ?
Arvixe/Elgg Community Liaison