Look, any writer worth their sand can figure out a way to effectively write Spider-man, Venom, Carnage and Lizard into a movie. Why, I've been doing this alot in my head at night while at work and I'm just a lousy fanfiction writer. What you need to find is a writer who is adept at ensemble pieces and sufficiently rotating the characters so that everyone gets equal airtime. In my head I've created a scenario where Peter finds out Brock is still very much alive, perhaps have Eddie as Venom terrorize Peter a bit. Then mysterious, brutal murders start taking place and the blame immediately falls on Venom. Add into this the origin of Lizard since we do not need to set up Venom's origin since this was done in SM3. I was thinking Dr. Connors could show Peter how he's been able to create a serum that regenerates lost limbs utilizing lizard DNA. Perhaps throw in scene of the transmogrification later on of Connors changing into Lizard with Peter coming to his lab afterwards for some scientific consultation, he spots Connors in a corner writhing in agony but when he checks to see if Connors is okay, he jerks around to reveal he has become the Lizard, fights ensues, Connors bests Spidey and hides in the sewers, yadda yadda yadda. Then we get the reveal that Carnage is the real killer, absolving Eddie of guilt. Spidey and Venom team up in the final showdown to take out Carnage and rescue Dr. Connors with an antidote to reverse the serum. The End. Why is this so monumentally difficult to do in your eyes?! I'm an amateur and I easily figured things out in 5 seconds.
P.S. Why yes, I am available to help write a script for Spidey 4 too. Just come on down to the Laundrymat I work at at the beginning of each week when I'm scheduled to work.