So its now three atcks under 5th level, and 4 attacks at 5th level. One weapon has to be Nick, the other three just have to be light, so you could have a variety of weapon masteries there. With the Dual Wielder feat, you don't need to use Light anymore. Therefore, it seems to me Nick is a combo-starter; you start with Nick and that opens up a range of different weapons you can produce to knock your opponent around, sap them, vex them, etc etc.
On all single class characters except Monks and Fighters it is 2 attacks a round level 4 and under if using 2 light weapons. The only thing that changes is if you have Nick on one of the weapons you don't need to use a bonus action.
On a single class Fighter it is 2 attacks or 3 when you action surge*
On a single class Monk it is 2 attacks or 3 when you FOB.
If you multiclass you can raise this number with one level in Monk OR 3 levels in Dance Bard OR 3 levels in War Cleric along with a class that gets weapon mastery (Barbarian, Paladin, Fighter, Ranger, Rogue).
If you have a 2+ Monk levels and the nick property and weapon mastery from a one level dip, you would get 3 attacks (action, nick attack, martial arts attack) or 4 attacks when you use FOB at 4th leve.
If you have 3 levels in dance bard plus one level for mastery you could get 3 attacks (action, nick, martial arts) if you used a bardic inspiration that turn
If you have 3 levels of War Cleric you could get 3 attacks (action, nick, war priest) a limited number of times.
I believe the maximum number of attacks in a round at level 4 is 5 attacks on a Fighter 2-Monk 2. Attack, Action Surge Attack, Nick, FOB.
I could be wrong on this. This is listening to some of the videos and interpreting what they said.
* Edit - After consideration I think you can get 4 attacks in a single round with a level 4 fighter if you start the turn holding 2 light weapons (one with nick) and have the Pole Arm Master feat: Attack - Nick - drop light weapons and pull out pole arm - action surge attack - PAM Bonus action attack. You could only do this the first round of combat though.