Three of them are related, so they'll always be tied together. One shouldn't ONLY be defined by their family though. Or limited to only being on the one team.

X-Men go solo all the time, making the four split up is about the only thing they haven't done in the comics till now and that's just cause half of them are dead or in limbo. The Invisible Woman working for SHIELD was an incredible idea that was wasted when she was written off with Reed. The Thing in the GotG was a nice change. And it was about time the actual human Torch joined the Avengers and not just the machine again. I just think there's more potential when you split them up and move some characters around. If you've read one or two F4 stories there's a good chance you know how every other story played out afterwards. 

The only thing the F4 really did was introduce other characters like the Inhumans, the Skrulls, the Kree, Black Panther, etc. Aside from the Hickman run before the FF title the comics had been dropping in sales and the team didn't seem to matter in the grande scheme that is Marvel. To me it seemed like fans stopped caring around the same time Marvel did.

~KL~