Both Spy Kids and Scream are extremely solid franchises. Each film was a success and more than doubled its budget. If you stack all 6 films from both series together it's about one billion dollars worldwide. That's quite impressive. Star Wars, Indiana Jones and Lord of the Rings are exceptions to the rule. Most films don't fare nearly as well as these franchises so if 7 to 10 years later the creators feel there's more to say then I say go for it. It's not like the Nightmare on Elm Street reboot that watered down the original and was done without any of the original crew. That actually did well enough for a sequel and they're making one. Ick. That's cynical film making.
There is call for Scream to do well (again, if it's released at the right time, any horror movie will do well with the jock crowd), but I don't see Spy Kids being anything but Home Alone 3. The kids have grown up (Alexa especially... I'm becoming a dirty old man) and I have yet to see a new class movie hold up to the original original crew or no original crew. Plus, Cars 2 seems to be another Spy based movie (unfortunately) so that is a gamble. But again, if these do indeed prove to still be strong, it might just light a fire under FR and get it going again. IF they're indeed successful. They'd have to be huge at that, and even if they do make the big bucks, I doubt they'll make the big BIG big bucks.
That said, I gotta say, I dug the first Spy Kids film on TV... didn't see the second or third... (though the time is right for the 3rd on to be on Blu rereleased in 3-D since it's the go to gimmick). But something tells me Spy Kids REALLY deserved a Saturday Morning Cartoon. Why Disney didn't do one is beyond me... even DIC could have got one out there (why they did an Olsen Twins one was beyond ANYONE). A fourth movie, not quite old enough to be nostalgic, not young enough to be a smooth transition is a risky bit. Especially given how competitive next Summer's gonna be... For kid's movies alone we got the second Cars and Kung Fu Pandas not to mention a new Pooh (and I HOPE that does well). Something tells me the fate of FR may just rest on SK4... obviously since they're kid's films.