If you reduce a team from 7 to 4, especially if the reason is something outside the team, the best course of action is probably to treat this as a completely new team. People will need to change into new roles, find a new way of working together, cover fresh weaknesses caused by those leaving, find out who is in charge of what, and all the other problems that a new team runs in to.
Cutting a team in half shouldn't be handled in a "business as usual" type of way; it's a pretty traumatic experience for the team, especially if they've been working together for a while. You could go all the way and run a new team introduction, but at the very least you should give them some time to reform and whatever velocity you have from the previous team can probably go out the window.
Doing whatever it is you normally do when planning with a new team will likely give the most reliable results. (The most reliable possible, that is. They probably won't be very reliable regardless.)