Typically with a 4th official he will record everything, but normally the senior AR will also record all cautions and misconduct, sometimes with junior AR recording goals.
These things probably need covering, over and above what you'd do for a team of 3 briefing.
- roles and responsibilities if one official goes down. Usually if the referee is injured the 4th official takes over, but sometimes one of the ARs will be senior to the 4th official so he would take over instead.
- what you want the 4th official to record. I would get him to record absolutely everything, including a stoppages log. Every time there is a stoppage, injuries, subs, lost ball, generally anything 30 seconds or more, get him to note it down (time taken and reason), that way if there are complaints about how much or little added time you play you have evidence to throw back at them.
- who does what at substitutions. Should AR1 always come and help out or should that only be if there or 2 or more changes. If AR1 is helping who does the kit check. Assuming you don't have comms, who gets the signal to you for a sub, typically AR1 would get told and then buzz the referee. If you don't have buzzers 4th man would tell AR1 to raise flag, but realistically you'd both be shouting "ref, sub".
- how do you want the benches managed. Should he keep warning about too many standing, or just do it a few times and then give up and report them after the game. What do you want around managing behaviour, will you come over for a chat or if you are coming over does that mean a card is coming out. Remember the clubs probably won't be used to technical areas, let alone a 4th official, and it isn't uncommon for them to look like bus shelters. Regardless of standing numbers, make sure only those named are in there, and make sure no one under 16 is in there. If you have team sheets and then take action against someone in the TA who shouldn't be there it will not look good if it goes to a hearing. If the team sheet doesn't have a section for TA members get them to write the names on the back.
- what happens at half time and full time. I would ask my 4th official to get straight into the tunnel as soon as possible after full time in case anything happens, some referees don't like that and would rather they stay at the edge and met them as they go in (on the basis if you don't see it kicking off you don't have to deal with it). Not so relevant at the end of a cup game as you would hope everyone would stay out for presentations.
- If it will go to penalties after a draw task the 4th official with ensuring that the 11 that finish the game stay on the pitch and none of the subs go on. Even better get him to note the 22 numbers on just before the end of ET (or FT if no ET). This can be tricky, especially with rolling subs, plenty of cup games have had to be replayed due to players who weren't on at the end taking penalties. You really don't want this to happen to you in a cup final.
- probably most important, tell them not to defend the indefensible. Being 4th official can be a very lonely place, you can't run away like the referee and (sometimes) AR1 can, you are stuck there and if you try and defend every decision it can be a very painful 90 minutes. As long as it doesn't relate to a goal, penalty, attacking free kick, etc, it is fine to say something like "from here I can see what you are saying but the referee is looking at it from a different angle".
Above all, enjoy ...!
) that I like 4Os to help me with during a match.