Sorry for the late reply.
Okay, I hope I understood your question.
The crew system is pretty "static", meaning that you can invite 3 people to join your crew (if they accept the request sent to them), and then they will be visible on the left side of the screen indicating that these players are in your crew, now, if your crew members are online - their names will be "non-transparent", and if they are offline, their names are "transparent" as to indicate that they are offline . - So from the build I have played, there are no crew list from which players can step in the crew in case of a "free spot" in your crew (simply because an offline player isn't freeing up a spot in the crew, and no slots are available until a player actively leaves the crew). You have a friends list, but tbh I haven't really experimented too much with that particular side of it.
Also, now that I am writing about it, I don't remember if a player needs to be in your friendslist first before inviting him to the crew.
But I sure hope they will change it a bit.
The 4 player limit might be due to the overall limit of 8 players in the same session - so to make it possible for 2 crews to battle, pvp, or something else, I think they have limited it. - but yea, I see your point with the Forza comparison, and I totally agree, that would be very good. I mean even TDU2 had a decent "club system" with club houses and the ability to have 2 groups within the same club doing races simultaneously in order to further the overall rank of the club house.
Surely, some things can be improved, and I like the idea to have a "pool of players" to fill in vacancies in a crew.