Once your team has received a slot (either via automatic allocation, a wild-card pick or by qualifying via a Satellite) you will see a "Manage Roster" button for the related Challenger in the "Challenger Pre-Registration Overview".
Clicking this button will allow you to specify 4 players that will play in that specific event. Also you will need to provide travel details for each player.
Do note that players which can't be picked have mostlikely not yet accepted the nomination for your team (see also What kind of a status can a player have? for further reference)
Further you will be able to specify the roster of 4 players only once since the LOC will rely on the data (e.g. for organizing pickups / transfer)